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By mitsubishi · Posted
Online Trading Scams 4-6 minutes Home Features Online trading scams are a most specious type of fraud, combining the safety of online anonymity with the high personal importance of your investments. Imagine your 401(k) disappearing in a second because a shifty online presence claimed it could offer you enticing returns on investment. Here are a few examples of online stock trading scams so you can keep yourself out of hawk from financial fraud. Image 1 of 1 Fraudulent Pre-IPO Investing In this type of online trading scam, an investor buys a stake in a company before its initial public offering (IPO) of securities. There is a high level of risk in any pre-IPO investment, but an investor can face several pitfalls in this situation. First, the offering may be illegitimate. Any company that wants to sell securities to the public must file the transaction with the SEC or qualify for an exemption. If this has not happened, you could lose your total investment. Second, the company may never actually go public. Fraudsters may use the IPO's hype to lure investors and gain money without going public. The best way to avoid this type of online trading scam is similar to other scams: investigate. Knowing more about the company will give you better insight as to whether the offer is legitimate or fraudulent. Research the company. What services or products does it provide? Can you access the company's financials? Review the management team. A history of SEC investigations and unhappy investors is a bad sign. Take the time to do the homework and you'll find yourself in a safer position. Ponzi Schemes A good, old-fashioned Ponzi scheme is easier to pull off online these days, making it a common and lucrative online trading scam. A classic Ponzi scheme goes like this: An advertisement is placed that proclaims a "high-yield investment program" where investors buy stakes in a scheme that takes advantage of their lack of investing experience and knowledge. When the initial investment plus returns is dispersed, the investors are likely to reinvest. The returns, however, are actually the initial investments paid by new entrants, not profits made from real investments. Online, anonymity protects Ponzi-scheme promoters, making it easy for them to disappear when the scheme ends. This type of online trading scam preys on investor ignorance. Investors are lured by promises of high returns that appear real, and tricky transfers to new schemes give the illusion that the account is solvent. Keep out of this kind of scam by investigating the promoter. Find out if the investment is registered with the SEC. Arm yourself with knowledge about this type of scams. Remember, if it sounds too good to be true, it probably is. Pump and Dump Schemes This type of online trading scam leads to overvalued stocks from misleading statements and promises of insider tips. In a pump and dump scheme, a promoter issues a call for investors to buy a stock because the promoter knows something special about the company (the pump). Then, as investors buy into a stock and inflate the price, the fraudsters cash out on their original shares (the dump). This type of scam is commonly seen on the internet, where message boards can catch fire with promises of fast returns and insider info. When the fraudsters dump their stocks, the average investors lose their money as the stock devalues quickly. As with any other online trading scam, remain skeptical. Don't buy into an investment too rashly, and take standard analysis like company profiles and financial statements into account. Investigate the promoter. What does he have to gain from your investment? Verify claims independently; don't rely on one source. If you believe you've stumbled upon an online trading scam, don't invest. Take a moment to view the fund or investment from all angles and determine whether you've found a legitimate venture. Check credentials and research sources. The best way to avoid an online trading scam is to be knowledgeable. At TopTenREVIEWS We Do the Research So You Don't Have To. Disclaimer: We provide this information as an analysis of the most up-to-date sources, but we do not attempt to interpret SEC regulations. If you need any clarifications on any of the laws or rules presented in this article, it's best to notify an attorney who specializes in securities law. -
By mitsubishi · Posted
Due to overwhelming demand.. Adhesion From Wikipedia, the free encyclopedia Jump to navigation Jump to search For other uses, see Adhesion (disambiguation). Dew drops adhering to a spider web Play media Adhesion of a frog on a wet vertical glass surface. IUPAC definition Process of attachment of a substance to the surface of another substance. Note 1: Adhesion requires energy that can come from chemical and/or physical linkages, the latter being reversible when enough energy is applied. Note 2: In biology, adhesion reflects the behavior of cells shortly after contact to the surface. Note 3: In surgery, adhesion is used when two tissues fuse unexpectedly.[1] Adhesion is the tendency of dissimilar particles or surfaces to cling to one another (cohesion refers to the tendency of similar or identical particles/surfaces to cling to one another). The forces that cause adhesion and cohesion can be divided into several types. The intermolecular forces responsible for the function of various kinds of stickers and sticky tape fall into the categories of chemical adhesion, dispersive adhesion, and diffusive adhesion. In addition to the cumulative magnitudes of these intermolecular forces, there are also certain emergent mechanical effects. Contents 1 Surface energy 2 Mechanisms 2.1 Mechanical 2.2 Chemical 2.3 Dispersive 2.4 Electrostatic 2.5 Diffusive 3 Strength 4 Other effects 4.1 Stringing 4.2 Microstructures 4.3 Hysteresis 4.4 Wettability and adsorption 4.5 Lateral adhesion 5 See also 6 References 7 Further reading Surface energy Diagram of various cases of cleavage, with each unique species labeled. A: γ = (1/2)W11 B: W12 = γ1 + γ2 – γ12 C: γ12 = (1/2)W121 = (1/2)W212 D: W12 + W33 – W13 – W23 = W132. Surface energy is conventionally defined as the work that is required to build an area of a particular surface. Another way to view the surface energy is to relate it to the work required to cleave a bulk sample, creating two surfaces. If the new surfaces are identical, the surface energy γ of each surface is equal to half the work of cleavage, W: γ = (1/2)W11. If the surfaces are unequal, the Young-Dupré equation applies: W12 = γ1 + γ2 – γ12, where γ1 and γ2 are the surface energies of the two new surfaces, and γ12 is the interfacial energy. This methodology can also be used to discuss cleavage that happens in another medium: γ12 = (1/2)W121 = (1/2)W212. These two energy quantities refer to the energy that is needed to cleave one species into two pieces while it is contained in a medium of the other species. Likewise for a three species system: γ13 + γ23 – γ12 = W12 + W33 – W13 – W23 = W132, where W132 is the energy of cleaving species 1 from species 2 in a medium of species 3.[2] A basic understanding of the terminology of cleavage energy, surface energy, and surface tension is very helpful for understanding the physical state and the events that happen at a given surface, but as discussed below, the theory of these variables also yields some interesting effects that concern the practicality of adhesive surfaces in relation to their surroundings.[2] Mechanisms There is no single theory covering adhesion, and particular mechanisms are specific to particular material scenarios. Five mechanisms of adhesion have been proposed to explain why one material sticks to another: Mechanical Adhesive materials fill the voids or pores of the surfaces and hold surfaces together by interlocking. Other interlocking phenomena are observed on different length scales. Sewing is an example of two materials forming a large scale mechanical bond, velcro forms one on a medium scale, and some textile adhesives (glue) form one at a small scale. Chemical Two materials may form a compound at the joint. The strongest joints are where atoms of the two materials share or swap electrons (known respectively as covalent bonding or ionic bonding). A weaker bond is formed if a hydrogen atom in one molecule is attracted to an atom of nitrogen, oxygen, or fluorine in another molecule, a phenomenon called hydrogen bonding. Chemical adhesion occurs when the surface atoms of two separate surfaces form ionic, covalent, or hydrogen bonds. The engineering principle behind chemical adhesion in this sense is fairly straightforward: if surface molecules can bond, then the surfaces will be bonded together by a network of these bonds. It bears mentioning that these attractive ionic and covalent forces are effective over only very small distances – less than a nanometer. This means in general not only that surfaces with the potential for chemical bonding need to be brought very close together, but also that these bonds are fairly brittle, since the surfaces then need to be kept close together.[3] Dispersive Main article: Dispersive adhesion In dispersive adhesion, also known as physisorption, two materials are held together by van der Waals forces: the attraction between two molecules, each of which has a region of slight positive and negative charge. In the simple case, such molecules are therefore polar with respect to average charge density, although in larger or more complex molecules, there may be multiple "poles" or regions of greater positive or negative charge. These positive and negative poles may be a permanent property of a molecule (Keesom forces) or a transient effect which can occur in any molecule, as the random movement of electrons within the molecules may result in a temporary concentration of electrons in one region (London forces). Cohesion causes water to form drops, surface tension causes them to be nearly spherical, and adhesion keeps the drops in place. Water droplets are flatter on a Hibiscus flower which shows better adhesion. In surface science, the term adhesion almost always refers to dispersive adhesion. In a typical solid-liquid-gas system (such as a drop of liquid on a solid surrounded by air) the contact angle is used to evaluate adhesiveness indirectly, while a Centrifugal Adhesion Balance allows for direct quantitative adhesion measurements. Generally, cases where the contact angle is low are considered of higher adhesion per unit area. This approach assumes that the lower contact angle corresponds to a higher surface energy.[4] Theoretically, the more exact relation between contact angle and work of adhesion is more involved and is given by the Young-Dupre equation. The contact angle of the three-phase system is a function not only of dispersive adhesion (interaction between the molecules in the liquid and the molecules in the solid) but also cohesion (interaction between the liquid molecules themselves). Strong adhesion and weak cohesion results in a high degree of wetting, a lyophilic condition with low measured contact angles. Conversely, weak adhesion and strong cohesion results in lyophobic conditions with high measured contact angles and poor wetting. London dispersion forces are particularly useful for the function of adhesive devices, because they don't require either surface to have any permanent polarity. They were described in the 1930s by Fritz London, and have been observed by many researchers. Dispersive forces are a consequence of statistical quantum mechanics. London theorized that attractive forces between molecules that cannot be explained by ionic or covalent interaction can be caused by polar moments within molecules. Multipoles could account for attraction between molecules having permanent multipole moments that participate in electrostatic interaction. However, experimental data showed that many of the compounds observed to experience van der Waals forces had no multipoles at all. London suggested that momentary dipoles are induced purely by virtue of molecules being in proximity to one another. By solving the quantum mechanical system of two electrons as harmonic oscillators at some finite distance from one another, being displaced about their respective rest positions and interacting with each other's fields, London showed that the energy of this system is given by: E=3hν−34hνα2R6{\displaystyle E=3h\nu -{\frac {3}{4}}{\frac {h\nu \alpha ^{2}}{R^{6}}}} While the first term is simply the zero-point energy, the negative second term describes an attractive force between neighboring oscillators. The same argument can also be extended to a large number of coupled oscillators, and thus skirts issues that would negate the large scale attractive effects of permanent dipoles cancelling through symmetry, in particular. The additive nature of the dispersion effect has another useful consequence. Consider a single such dispersive dipole, referred to as the origin dipole. Since any origin dipole is inherently oriented so as to be attracted to the adjacent dipoles it induces, while the other, more distant dipoles are not correlated with the original dipole by any phase relation (thus on average contributing nothing), there is a net attractive force in a bulk of such particles. When considering identical particles, this is called cohesive force.[5] When discussing adhesion, this theory needs to be converted into terms relating to surfaces. If there is a net attractive energy of cohesion in a bulk of similar molecules, then cleaving this bulk to produce two surfaces will yield surfaces with a dispersive surface energy, since the form of the energy remain the same. This theory provides a basis for the existence of van der Waals forces at the surface, which exist between any molecules having electrons. These forces are easily observed through the spontaneous jumping of smooth surfaces into contact. Smooth surfaces of mica, gold, various polymers and solid gelatin solutions do not stay apart when their separating becomes small enough – on the order of 1–10 nm. The equation describing these attractions was predicted in the 1930s by De Boer and Hamaker:[3] Parea=−A24πz3{\displaystyle {\frac {P}{area}}=-{\frac {A}{24\pi z^{3}}}} where P is the force (negative for attraction), z is the separation distance, and A is a material-specific constant called the Hamaker constant. The two stages of PDMS microstructure collapse due to van der Waals attractions. The PDMS stamp is indicated by the hatched region, and the substrate is indicated by the shaded region. A) The PDMS stamp is placed on a substrate with the "roof" elevated. B) Van der Waals attractions make roof collapse energetically favorable for PDMS stamp. The effect is also apparent in experiments where a polydimethylsiloxane (PDMS) stamp is made with small periodic post structures. The surface with the posts is placed face down on a smooth surface, such that the surface area in between each post is elevated above the smooth surface, like a roof supported by columns. Because of these attractive dispersive forces between the PDMS and the smooth substrate, the elevated surface – or “roof” – collapses down onto the substrate without any external force aside from the van der Waals attraction.[6] Simple smooth polymer surfaces – without any microstructures – are commonly used for these dispersive adhesive properties. Decals and stickers that adhere to glass without using any chemical adhesives are fairly common as toys and decorations and useful as removable labels because they do not rapidly lose their adhesive properties, as do sticky tapes that use adhesive chemical compounds. It is important to note that these forces also act over very small distances – 99% of the work necessary to break van der Waals bonds is done once surfaces are pulled more than a nanometer apart.[3] As a result of this limited motion in both the van der Waals and ionic/covalent bonding situations, practical effectiveness of adhesion due to either or both of these interactions leaves much to be desired. Once a crack is initiated, it propagates easily along the interface because of the brittle nature of the interfacial bonds.[7] As an additional consequence, increasing surface area often does little to enhance the strength of the adhesion in this situation. This follows from the aforementioned crack failure – the stress at the interface is not uniformly distributed, but rather concentrated at the area of failure.[3] Electrostatic Some conducting materials may pass electrons to form a difference in electrical charge at the joint. This results in a structure similar to a capacitor and creates an attractive electrostatic force between the materials. Diffusive Some materials may merge at the joint by diffusion. This may occur when the molecules of both materials are mobile and soluble in each other. This would be particularly effective with polymer chains where one end of the molecule diffuses into the other material. It is also the mechanism involved in sintering. When metal or ceramic powders are pressed together and heated, atoms diffuse from one particle to the next. This joins the particles into one. The interface is indicated by the dotted line. A) Non-crosslinked polymers are somewhat free to diffuse across the interface. One loop and two distal tails are seen diffusing. B) Crosslinked polymers not free enough to diffuse. C) "Scissed" polymers very free, with many tails extending across the interface. Diffusive forces are somewhat like mechanical tethering at the molecular level. Diffusive bonding occurs when species from one surface penetrate into an adjacent surface while still being bound to the phase of their surface of origin. One instructive example is that of polymer-on-polymer surfaces. Diffusive bonding in polymer-on-polymer surfaces is the result of sections of polymer chains from one surface interdigitating with those of an adjacent surface. The freedom of movement of the polymers has a strong effect on their ability to interdigitate, and hence, on diffusive bonding. For example, cross-linked polymers are less capable of diffusion and interdigitation because they are bonded together at many points of contact, and are not free to twist into the adjacent surface. Uncrosslinked polymers (thermoplastics), on the other hand are freer to wander into the adjacent phase by extending tails and loops across the interface. Another circumstance under which diffusive bonding occurs is “scission”. Chain scission is the cutting up of polymer chains, resulting in a higher concentration of distal tails. The heightened concentration of these chain ends gives rise to a heightened concentration of polymer tails extending across the interface. Scission is easily achieved by ultraviolet irradiation in the presence of oxygen gas, which suggests that adhesive devices employing diffusive bonding actually benefit from prolonged exposure to heat/light and air. The longer such a device is exposed to these conditions, the more tails are scissed and branch out across the interface. Once across the interface, the tails and loops form whatever bonds are favorable. In the case of polymer-on-polymer surfaces, this means more van der Waals forces. While these may be brittle, they are quite strong when a large network of these bonds is formed. The outermost layer of each surface plays a crucial role in the adhesive properties of such interfaces, as even a tiny amount of interdigitation – as little as one or two tails of 1.25 angstrom length – can increase the van der Waals bonds by an order of magnitude.[8] Strength The strength of the adhesion between two materials depends on which of the above mechanisms occur between the two materials, and the surface area over which the two materials contact. Materials that wet against each other tend to have a larger contact area than those that do not. Wetting depends on the surface energy of the materials. Low surface energy materials such as polyethylene, polypropylene, polytetrafluoroethylene and polyoxymethylene are difficult to bond without special surface preparation. Another factor determining the strength of an adhesive contact is its shape. Adhesive contacts of complex shape begin to detach at the "edges" of the contact area[9]. The process of destruction of adhesive contacts can be seen in the film[10]. Other effects In concert with the primary surface forces described above, there are several circumstantial effects in play. While the forces themselves each contribute to the magnitude of the adhesion between the surfaces, the following play a crucial role in the overall strength and reliability of an adhesive device. Stringing Fingering process. The hatched area is the receiving substrate, the dotted strip is the tape, and the shaded area in between is the adhesive chemical layer. The arrow indicates the direction of propagation for the fracture. Stringing is perhaps the most crucial of these effects, and is often seen on adhesive tapes. Stringing occurs when a separation of two surfaces is beginning and molecules at the interface bridge out across the gap, rather than cracking like the interface itself. The most significant consequence of this effect is the restraint of the crack. By providing the otherwise brittle interfacial bonds with some flexibility, the molecules that are stringing across the gap can stop the crack from propagating.[3] Another way to understand this phenomenon is by comparing it to the stress concentration at the point of failure mentioned earlier. Since the stress is now spread out over some area, the stress at any given point has less of a chance of overwhelming the total adhesive force between the surfaces. If failure does occur at an interface containing a viscoelastic adhesive agent, and a crack does propagate, it happens by a gradual process called “fingering”, rather than a rapid, brittle fracture.[7] Stringing can apply to both the diffusive bonding regime and the chemical bonding regime. The strings of molecules bridging across the gap would either be the molecules that had earlier diffused across the interface or the viscoelastic adhesive, provided that there was a significant volume of it at the interface. Microstructures The interplay of molecular scale mechanisms and hierarchical surface structures is known to result in high levels of static friction and bonding between pairs of surfaces [11]. Technologically advanced adhesive devices sometimes make use of microstructures on surfaces, such as tightly packed periodic posts. These are biomimetic technologies inspired by the adhesive abilities of the feet of various arthropods and vertebrates (most notably, geckos). By intermixing periodic breaks into smooth, adhesive surfaces, the interface acquires valuable crack-arresting properties. Because crack initiation requires much greater stress than does crack propagation, surfaces like these are much harder to separate, as a new crack has to be restarted every time the next individual microstructure is reached.[12] Hysteresis Hysteresis, in this case, refers to the restructuring of the adhesive interface over some period of time, with the result being that the work needed to separate two surfaces is greater than the work that was gained by bringing them together (W > γ1 + γ2). For the most part, this is a phenomenon associated with diffusive bonding. The more time is given for a pair of surfaces exhibiting diffusive bonding to restructure, the more diffusion will occur, the stronger the adhesion will become. The aforementioned reaction of certain polymer-on-polymer surfaces to ultraviolet radiation and oxygen gas is an instance of hysteresis, but it will also happen over time without those factors. In addition to being able to observe hysteresis by determining if W > γ1 + γ2 is true, one can also find evidence of it by performing “stop-start” measurements. In these experiments, two surfaces slide against one another continuously and occasionally stopped for some measured amount of time. Results from experiments on polymer-on-polymer surfaces show that if the stopping time is short enough, resumption of smooth sliding is easy. If, however, the stopping time exceeds some limit, there is an initial increase of resistance to motion, indicating that the stopping time was sufficient for the surfaces to restructure.[8] Wettability and adsorption Some atmospheric effects on the functionality of adhesive devices can be characterized by following the theory of surface energy and interfacial tension. It is known that γ12 = (1/2)W121 = (1/2)W212. If γ12 is high, then each species finds it favorable to cohere while in contact with a foreign species, rather than dissociate and mix with the other. If this is true, then it follows that when the interfacial tension is high, the force of adhesion is weak, since each species does not find it favorable to bond to the other. The interfacial tension of a liquid and a solid is directly related to the liquid's wettability (relative to the solid), and thus one can extrapolate that cohesion increases in non-wetting liquids and decreases in wetting liquids. One example that verifies this is polydimethyl siloxane rubber, which has a work of self-adhesion of 43.6 mJ/m2 in air, 74 mJ/m2 in water (a nonwetting liquid) and 6 mJ/m2 in methanol (a wetting liquid). This argument can be extended to the idea that when a surface is in a medium with which binding is favorable, it will be less likely to adhere to another surface, since the medium is taking up the potential sites on the surface that would otherwise be available to adhere to another surface. Naturally this applies very strongly to wetting liquids, but also to gas molecules that could adsorb onto the surface in question, thereby occupying potential adhesion sites. This last point is actually fairly intuitive: Leaving an adhesive exposed to air too long gets it dirty, and its adhesive strength will decrease. This is observed in the experiment: when mica is cleaved in air, its cleavage energy, W121 or Wmica/air/mica, is smaller than the cleavage energy in vacuum, Wmica/vac/mica, by a factor of 13.[3] Lateral adhesion Lateral adhesion is the adhesion associated with sliding one object on a substrate such as sliding a drop on a surface. When the two objects are solids, either with or without a liquid between them, the lateral adhesion is described as friction. However, the behavior of lateral adhesion between a drop and a surface is tribologically very different from friction between solids, and the naturally adhesive contact between a flat surface and a liquid drop makes the lateral adhesion in this case, an individual field. Lateral adhesion can be measured using the centrifugal adhesion balance (CAB),[13][14] which uses a combination of centrifugal and gravitational forces to decouple the normal and lateral forces in the problem. See also Adhesive Adhesive bonding Bacterial adhesin Capillary action Cell adhesion Contact mechanics Fracture mechanics Galling Insect adhesion Meniscus Mucoadhesion Pressure-sensitive adhesive Rail adhesion Synthetic setae Cohesion number References Vert, Michel; Doi, Yoshiharu; Hellwich, Karl-Heinz; Hess, Michael; Hodge, Philip; Kubisa, Przemyslaw; Rinaudo, Marguerite; Schué, François (2012). "Terminology for biorelated polymers and applications (IUPAC Recommendations 2012)" (PDF). Pure and Applied Chemistry. 84 (2): 377–410. doi:10.1351/PAC-REC-10-12-04. J. N. Israelachvili, Intermolecular and Surface Forces (Academic Press, New York, 1985). chap. 15. K. Kendall (1994). "Adhesion: Molecules and Mechanics". Science. 263 (5154): 1720–5. doi:10.1126/science.263.5154.1720. PMID 17795378. Laurén, Susanna. "What is required for good adhesion?". blog.biolinscientific.com. Retrieved 2019-12-31. F. London, "The General Theory of Molecular Forces" (1936). Y. Y. Huang; Zhou, Weixing; Hsia, K. J.; Menard, Etienne; Park, Jang-Ung; Rogers, John A.; Alleyne, Andrew G. (2005). "Stamp Collapse in Soft Lithography" (PDF). Langmuir. 21 (17): 8058–68. doi:10.1021/la0502185. PMID 16089420. Bi-min Zhang Newby, Manoj K. Chaudhury and Hugh R. Brown (1995). "Macroscopic Evidence of the Effect of Interfacial Slippage on Adhesion" (PDF). Science. 269 (5229): 1407–9. doi:10.1126/science.269.5229.1407. PMID 17731150. N. Maeda; Chen, N; Tirrell, M; Israelachvili, JN (2002). "Adhesion and Friction Mechanisms of Polymer-on-Polymer Surfaces". Science. 297 (5580): 379–82. doi:10.1126/science.1072378. PMID 12130780. Popov, Valentin L.; Pohrt, Roman; Li, Qiang (2017-09-01). "Strength of adhesive contacts: Influence of contact geometry and material gradients". Friction. 5 (3): 308–325. doi:10.1007/s40544-017-0177-3. ISSN 2223-7690. Friction Physics (2017-12-06), Science friction: Adhesion of complex shapes, retrieved 2017-12-30 Static Friction at Fractal Interfaces Tribology International 2016, Volume 93 A. Majmuder; Ghatak, A.; Sharma, A. (2007). "Microfluidic Adhesion Induced by Subsurface Microstructures". Science. 318 (5848): 258–61. doi:10.1126/science.1145839. PMID 17932295. Tadmor, Rafael (2009). "Measurement of Lateral Adhesion Forces at the Interface between a Liquid Drop and a Substrate". Physical Review Letters. 103 (26): 266101. doi:10.1103/physrevlett.103.266101. PMID 20366322. Tadmor, Rafael; Das, Ratul; Gulec, Semih; Liu, Jie; E. N’guessan, Hartmann; Shah, Meet; S. Wasnik, Priyanka; Yadav, Sakshi B. (2017-04-18). "Solid–Liquid Work of Adhesion". Langmuir. 33 (15): 3594–3600. doi:10.1021/acs.langmuir.6b04437. ISSN 0743-7463. PMID 28121158. Further reading -
By mitsubishi · Posted
117 LESSONS AND 33 other lessons FOREX traders can use to learn Forex posting genius And lead the pied pips piper into your wallet dont forget to like and subscribe Visual impairment From Wikipedia, the free encyclopedia (Redirected from Blind man) Jump to navigation Jump to search "Blindness" redirects here. For other uses, see Blindness (disambiguation). Visual impairment Other names Vision impairment, vision loss A white cane, the international symbol of blindness Specialty Ophthalmology Symptoms Decreased ability to see[1][2] Complications Non-24-hour sleep–wake disorder[3] Causes Uncorrected refractive errors, cataracts, glaucoma[4] Diagnostic method Eye examination[2] Treatment Vision rehabilitation, changes in the environment, assistive devices (eyeglasses, white cane)[2] Frequency 940 million / 13% (2015)[5] Visual impairment, also known as vision impairment or vision loss, is a decreased ability to see to a degree that causes problems not fixable by usual means, such as glasses.[1][2] Some also include those who have a decreased ability to see because they do not have access to glasses or contact lenses.[1] Visual impairment is often defined as a best corrected visual acuity of worse than either 20/40 or 20/60.[6] The term blindness is used for complete or nearly complete vision loss.[6] Visual impairment may cause people difficulties with normal daily activities such as driving, reading, socializing, and walking.[2] The most common causes of visual impairment globally are uncorrected refractive errors (43%), cataracts (33%), and glaucoma (2%).[4] Refractive errors include near-sightedness, far-sightedness, presbyopia, and astigmatism.[4] Cataracts are the most common cause of blindness.[4] Other disorders that may cause visual problems include age-related macular degeneration, diabetic retinopathy, corneal clouding, childhood blindness, and a number of infections.[7] Visual impairment can also be caused by problems in the brain due to stroke, premature birth, or trauma among others.[8] These cases are known as cortical visual impairment.[8] Screening for vision problems in children may improve future vision and educational achievement.[9] Screening adults without symptoms is of uncertain benefit.[10] Diagnosis is by an eye exam.[2] The World Health Organization (WHO) estimates that 80% of visual impairment is either preventable or curable with treatment.[4] This includes cataracts, the infections river blindness and trachoma, glaucoma, diabetic retinopathy, uncorrected refractive errors, and some cases of childhood blindness.[11] Many people with significant visual impairment benefit from vision rehabilitation, changes in their environment, and assistive devices.[2] As of 2015 there were 940 million people with some degree of vision loss.[5] 246 million had low vision and 39 million were blind.[4] The majority of people with poor vision are in the developing world and are over the age of 50 years.[4] Rates of visual impairment have decreased since the 1990s.[4] Visual impairments have considerable economic costs both directly due to the cost of treatment and indirectly due to decreased ability to work.[12] Contents 1 Classification 1.1 United Kingdom 1.2 United States 2 Health effects 2.1 Associated problems 3 Cause 3.1 Cataracts 3.2 Glaucoma 3.3 Infections 3.4 Injuries 3.5 Genetic defects 3.6 Poisoning 3.7 Other 4 Diagnosis 5 Prevention 6 Management 6.1 Mobility 6.2 Reading and magnification 6.3 Computers and mobile technology 6.4 Other aids and techniques 6.5 Communication 6.6 Healthcare access 7 Epidemiology 8 Society and culture 8.1 Legal definition 8.2 Literature and art 8.3 Sports 8.4 Metaphorical uses 9 Research 10 Other animals 11 See also 12 References 13 External links Classification A typical Snellen chart that is frequently used for visual acuity testing. The definition of visual impairment is reduced vision not corrected by glasses or contact lenses. The World Health Organization uses the following classifications of visual impairment. When the vision in the better eye with best possible glasses correction is: 20/30 to 20/60 : is considered mild vision loss, or near-normal vision 20/70 to 20/160 : is considered moderate visual impairment, or moderate low vision 20/200 to 20/400 : is considered severe visual impairment, or severe low vision 20/500 to 20/1,000 : is considered profound visual impairment, or profound low vision More than 20/1,000 : is considered near-total visual impairment, or near total blindness No light perception (NLP) : is considered total visual impairment, or total blindness Blindness is defined by the World Health Organization as vision in a person's best eye with best correction of less than 20/500 or a visual field of less than 10 degrees.[6] This definition was set in 1972, and there is ongoing discussion as to whether it should be altered to officially include uncorrected refractive errors.[1] United Kingdom Severely sight impaired Defined as having central visual acuity of less than 3/60 with normal fields of vision, or gross visual field restriction. Unable to see at 3 metres (10 ft) what the normally sighted person sees at 60 metres (200 ft). Sight impaired Able to see at 3 metres (10 ft), but not at 6 metres (20 ft), what the normally sighted person sees at 60 metres (200 ft) Less severe visual impairment is not captured by registration data, and its prevalence is difficult to quantify Low vision A visual acuity of less than 6/18 but greater than 3/60. Not eligible to drive and may have difficulty recognising faces across a street, watching television, or choosing clean, unstained, co-ordinated clothing.[13] In the UK, the Certificate of Vision Impairment (CVI) is used to certify patients as severely sight impaired or sight impaired.[14] The accompanying guidance for clinical staff states: "The National Assistance Act 1948 states that a person can be certified as severely sight impaired if they are "so blind as to be unable to perform any work for which eye sight is essential". Certification is based on whether a person can do any work for which eyesight is essential, not just one particular job (such as their job before becoming blind).[15] In practice, the definition depends on individuals' visual acuity and the extent to which their field of vision is restricted. The Department of Health identifies three groups of people who may be classified as severely visually impaired.[15] Those below 3/60 (equivalent to 20/400 in US notation) Snellen (most people below 3/60 are severely sight impaired). Those better than 3/60 but below 6/60 Snellen (people who have a very contracted field of vision only). Those 6/60 Snellen or above (people in this group who have a contracted field of vision especially if the contraction is in the lower part of the field). The Department of Health also state that a person is more likely to be classified as severely visually impaired if their eyesight has failed recently or if they are an older individual, both groups being perceived as less able to adapt to their vision loss.[15] United States In the United States, any person with vision that cannot be corrected to better than 20/200 in the best eye, or who has 20 degrees (diameter) or less of visual field remaining, is considered legally blind or eligible for disability classification and possible inclusion in certain government sponsored programs. In the United States, the terms partially sighted, low vision, legally blind and totally blind are used by schools, colleges, and other educational institutions to describe students with visual impairments.[16] They are defined as follows: Partially sighted indicates some type of visual problem, with a need of person to receive special education in some cases. Low vision generally refers to a severe visual impairment, not necessarily limited to distance vision. Low vision applies to all individuals with sight who are unable to read the newspaper at a normal viewing distance, even with the aid of eyeglasses or contact lenses. They use a combination of vision and other senses to learn, although they may require adaptations in lighting or the size of print, and, sometimes, Braille. Myopic – unable to see distant objects clearly, commonly called near-sighted or short-sighted. Hyperopic – unable to see close objects clearly, commonly called far-sighted or long-sighted. Legally blind indicates that a person has less than 20/200 vision in the better eye after best correction (contact lenses or glasses), or a field of vision of less than 20 degrees in the better eye. Totally blind students learn via Braille or other non-visual media. In 1934, the American Medical Association adopted the following definition of blindness: The United States Congress included this definition as part of the Aid to the Blind program in the Social Security Act passed in 1935.[17][18] In 1972, the Aid to the Blind program and two others combined under Title XVI of the Social Security Act to form the Supplemental Security Income program[19] which states: Health effects Visual impairments may take many forms and be of varying degrees. Visual acuity alone is not always a good predictor of the degree of problems a person may have. Someone with relatively good acuity (e.g., 20/40) can have difficulty with daily functioning, while someone with worse acuity (e.g., 20/200) may function reasonably well if their visual demands are not great. The American Medical Association has estimated that the loss of one eye equals 25% impairment of the visual system and 24% impairment of the whole person;[21][22] total loss of vision in both eyes is considered to be 100% visual impairment and 85% impairment of the whole person.[21] Some people who fall into this category can use their considerable residual vision – their remaining sight – to complete daily tasks without relying on alternative methods. The role of a low vision specialist (optometrist or ophthalmologist) is to maximize the functional level of a patient's vision by optical or non-optical means. Primarily, this is by use of magnification in the form of telescopic systems for distance vision and optical or electronic magnification for near tasks. People with significantly reduced acuity may benefit from training conducted by individuals trained in the provision of technical aids. Low vision rehabilitation professionals, some of whom are connected to an agency for the blind, can provide advice on lighting and contrast to maximize remaining vision. These professionals also have access to non-visual aids, and can instruct patients in their uses. The subjects making the most use of rehabilitation instruments, who lived alone, and preserved their own mobility and occupation were the least depressed, with the lowest risk of suicide and the highest level of social integration. Those with worsening sight and the prognosis of eventual blindness are at comparatively high risk of suicide and thus may be in need of supportive services. Many studies have demonstrated how rapid acceptance of the serious visual handicap has led to a better, more productive compliance with rehabilitation programs. Moreover, psychological distress has been reported to be at its highest when sight loss is not complete, but the prognosis is unfavorable. Therefore, early intervention is imperative for enabling successful psychological adjustment.[23] Associated problems Blindness can occur in combination with such conditions as intellectual disability, autism spectrum disorders, cerebral palsy, hearing impairments, and epilepsy.[24][25] Blindness in combination with hearing loss is known as deafblindness. It has been estimated that over half of completely blind people have non-24-hour sleep–wake disorder, a condition in which a person's circadian rhythm, normally slightly longer than 24 hours, is not entrained (synchronized) to the light–dark cycle.[26][27] Cause The most common causes of visual impairment globally in 2010 were: Refractive error (42%) Cataract (33%) Glaucoma (2%) Age-related macular degeneration (1%) Corneal opacification (1%) Diabetic retinopathy (1%) Childhood blindness Trachoma (1%) Undetermined (18%)[7] The most common causes of blindness worldwide in 2010 were: Cataracts (51%) Glaucoma (8%) Age-related macular degeneration (5%) Corneal opacification (4%) Childhood blindness (4%) Refractive errors (3%) Trachoma (3%) Diabetic retinopathy (1%) Undetermined (21%)[7] About 90% of people who are visually impaired live in the developing world.[4] Age-related macular degeneration, glaucoma, and diabetic retinopathy are the leading causes of blindness in the developed world.[28] Among working-age adults who are newly blind in England and Wales the most common causes in 2010 were:[29] Hereditary retinal disorders (20.2%) Diabetic retinopathy (14.4%) Optic atrophy (14.1%) Glaucoma (5.9%) Congenital abnormalities (5.1%) Disorders of the visual cortex (4.1%) Cerebrovascular disease (3.2%) Degeneration of the macula and posterior pole (3.0%) Myopia (2.8%) Corneal disorders (2.6%) Malignant neoplasms of the brain and nervous system (1.5%) Retinal detachment (1.4%) Cataracts Cataracts is the congenital and pediatric pathology that describes the greying or opacity of the crystalline lens, which is most commonly caused by intrauterine infections, metabolic disorders, and genetically transmitted syndromes.[30] Cataracts are the leading cause of child and adult blindness that doubles in prevalence with every ten years after the age of 40.[31] Consequently, today cataracts are more common among adults than in children.[30] That is, people face higher chances of developing cataracts as they age. Nonetheless, cataracts tend to have a greater financial and emotional toll upon children as they must undergo expensive diagnosis, long term rehabilitation, and visual assistance.[32] Also, according to the Saudi Journal for Health Sciences, sometimes patients experience irreversible amblyopia[30] after pediatric cataract surgery because the cataracts prevented the normal maturation of vision prior to operation.[33] Despite the great progress in treatment, cataracts remain a global problem in both economically developed and developing countries.[34] At present, with the variant outcomes as well as the unequal access to cataract surgery, the best way to reduce the risk of developing cataracts is to avoid smoking and extensive exposure to sun light (i.e. UV-B rays).[31] Glaucoma Glaucoma is a congenital and pediatric eye disease characterized by increased pressure within the eye or intraocular pressure (IOP).[35] Glaucoma causes visual field loss as well as severs the optic nerve.[36] Early diagnosis and treatment of glaucoma in patients is imperative because glaucoma is triggered by non-specific levels of IOP.[36] Also, another challenge in accurately diagnosing glaucoma is that the disease has four causes: 1) inflammatory ocular hypertension syndrome (IOHS); 2) severe uveitic angle closure; 3) corticosteroid-induced; and 4) a heterogonous mechanism associated with structural change and chronic inflammation.[35] In addition, often pediatric glaucoma differs greatly in cause and management from the glaucoma developed by adults.[37] Currently, the best sign of pediatric glaucoma is an IOP of 21 mm Hg or greater present within a child.[37] One of the most common causes of pediatric glaucoma is cataract removal surgery, which leads to an incidence rate of about 12.2% among infants and 58.7% among 10-year-olds.[37] Infections The burden of onchocerciasis: children leading blind adults in Africa Childhood blindness can be caused by conditions related to pregnancy, such as congenital rubella syndrome and retinopathy of prematurity. Leprosy and onchocerciasis each blind approximately 1 million individuals in the developing world. The number of individuals blind from trachoma has decreased in the past 10 years from 6 million to 1.3 million, putting it in seventh place on the list of causes of blindness worldwide. Central corneal ulceration is also a significant cause of monocular blindness worldwide, accounting for an estimated 850,000 cases of corneal blindness every year in the Indian subcontinent alone. As a result, corneal scarring from all causes is now the fourth greatest cause of global blindness.[38] Injuries Re-educating wounded. Blind French soldiers learning to make baskets, World War I. Eye injuries, most often occurring in people under 30, are the leading cause of monocular blindness (vision loss in one eye) throughout the United States. Injuries and cataracts affect the eye itself, while abnormalities such as optic nerve hypoplasia affect the nerve bundle that sends signals from the eye to the back of the brain, which can lead to decreased visual acuity. Cortical blindness results from injuries to the occipital lobe of the brain that prevent the brain from correctly receiving or interpreting signals from the optic nerve. Symptoms of cortical blindness vary greatly across individuals and may be more severe in periods of exhaustion or stress. It is common for people with cortical blindness to have poorer vision later in the day. Blinding has been used as an act of vengeance and torture in some instances, to deprive a person of a major sense by which they can navigate or interact within the world, act fully independently, and be aware of events surrounding them. An example from the classical realm is Oedipus, who gouges out his own eyes after realizing that he fulfilled the awful prophecy spoken of him. Having crushed the Bulgarians, the Byzantine Emperor Basil II blinded as many as 15,000 prisoners taken in the battle, before releasing them.[39] Contemporary examples include the addition of methods such as acid throwing as a form of disfigurement. Genetic defects People with albinism often have vision loss to the extent that many are legally blind, though few of them actually cannot see. Leber congenital amaurosis can cause total blindness or severe sight loss from birth or early childhood. Recent advances in mapping of the human genome have identified other genetic causes of low vision or blindness. One such example is Bardet–Biedl syndrome. Poisoning Rarely, blindness is caused by the intake of certain chemicals. A well-known example is methanol, which is only mildly toxic and minimally intoxicating, and breaks down into the substances formaldehyde and formic acid which in turn can cause blindness, an array of other health complications, and death.[40] When competing with ethanol for metabolism, ethanol is metabolized first, and the onset of toxicity is delayed. Methanol is commonly found in methylated spirits, denatured ethyl alcohol, to avoid paying taxes on selling ethanol intended for human consumption. Methylated spirits are sometimes used by alcoholics as a desperate and cheap substitute for regular ethanol alcoholic beverages. Other Amblyopia: is a category of vision loss or visual impairment that is caused by factors unrelated to refractive errors or coexisting ocular diseases.[41] Amblyopia is the condition when a child's visual systems fail to mature normally because the child either suffers from a premature birth, measles, congenital nubella syndrome, vitamin A deficiency, or meningitis.[42] If left untreated during childhood, amblyopia is currently incurable in adulthood because surgical treatment effectiveness changes as a child matures.[42] Consequently, amblyopia is the world's leading cause of child monocular vision loss, which is the damage or loss of vision in one eye.[41] In the best case scenario, which is very rare, properly treated amblyopia patients can regain 20/40 acuity.[41] Corneal opacification Degenerative myopia Diabetic retinopathy: is one of the manifestation microvascular complications of diabetes, which is characterized by blindness or reduced acuity. That is, diabetic retinopathy describes the retinal and vitreous hemorrhages or retinal capillary blockage caused by the increase of A1C,[43] which a measurement of blood glucose or sugar level.[44] In fact, as A1C increases, people tend to be at greater risk of developing diabetic retinopathy than developing other microvascular complications associated with diabetes (e.g. chronic hyperglycemia, diabetic neuropathy, and diabetic nephropathy).[43] Despite the fact that only 8% of adults 40 years and older experience vision-threatening diabetic retinopathy (e.g. nonproliferative diabetic retinopathy or NPDR and proliferative diabetic retinopathy or PDR), this eye diseased accounted for 17% of cases of blindness in 2002.[43] Retinitis pigmentosa Retinopathy of prematurity: The most common cause of blindness in infants worldwide. In its most severe form, ROP causes retinal detachment, with attendant visual loss. Treatment is aimed mainly at prevention, via laser or Avastin therapy. Stargardt's disease Uveitis: is a group of 30 intraocular inflammatory diseases[45] caused by infections, systemic diseases, organ-specific autoimmune processes, cancer or trauma.[46] That is, uveitis refers to a complex category of ocular diseases that can cause blindness if either left untreated or improperly diagnosed.[46] The current challenge of accurately diagnosing uveitis is that often the cause of a specific ocular inflammation is either unknown or multi-layered.[45] Consequently, about 3–10% uveitis victims in developed countries, and about 25% of victims in the developing countries, become blind from incorrect diagnosis and from ineffectual prescription of drugs, antibiotics or steroids.[46] In addition, uveitis is a diverse category of eye diseases that are subdivided as granulomatous (or tumorous) or non-granulomatous anterior, intermediate, posterior or pan uveitis.[46] In other words, uveitis diseases tend to be classified by their anatomic location in the eye (e.g. uveal tract, retina, or lens), as well as can create complication that can cause cataracts, glaucoma, retinal damage, age-related macular degeneration or diabetic retinopathy.[46] Xerophthalmia, often due to vitamin A deficiency, is estimated to affect 5 million children each year; 500,000 develop active corneal involvement, and half of these go blind. Diagnosis Play media Scientists track eye movements in glaucoma patients to check vision impairment while driving It is important that people be examined by someone specializing in low vision care prior to other rehabilitation training to rule out potential medical or surgical correction for the problem and to establish a careful baseline refraction and prescription of both normal and low vision glasses and optical aids. Only a doctor is qualified to evaluate visual functioning of a compromised visual system effectively.[47] The American Medical Association provides an approach to evaluating visual loss as it affects an individual's ability to perform activities of daily living.[21] Screening adults who have no symptoms is of uncertain benefit.[10] Prevention The World Health Organization estimates that 80% of visual loss is either preventable or curable with treatment.[4] This includes cataracts, onchocerciasis, trachoma, glaucoma, diabetic retinopathy, uncorrected refractive errors, and some cases of childhood blindness.[11] The Center for Disease Control and Prevention estimates that half of blindness in the United States is preventable.[2] Management Tommy Edison, a blind film critic, demonstrates for his viewers how a blind person can cook alone. Mobility Folded long cane A blind man is assisted by a guide dog in Brasília, Brazil Blind girl feels shape of vehicle near Mana village, Uttarakhand Visually impaired girl negotiating a rock while rock climbing Many people with serious visual impairments can travel independently, using a wide range of tools and techniques. Orientation and mobility specialists are professionals who are specifically trained to teach people with visual impairments how to travel safely, confidently, and independently in the home and the community. These professionals can also help blind people to practice travelling on specific routes which they may use often, such as the route from one's house to a convenience store. Becoming familiar with an environment or route can make it much easier for a blind person to navigate successfully. Tools such as the white cane with a red tip – the international symbol of blindness – may also be used to improve mobility. A long cane is used to extend the user's range of touch sensation. It is usually swung in a low sweeping motion, across the intended path of travel, to detect obstacles. However, techniques for cane travel can vary depending on the user and/or the situation. Some visually impaired persons do not carry these kinds of canes, opting instead for the shorter, lighter identification (ID) cane. Still others require a support cane. The choice depends on the individual's vision, motivation, and other factors. A small number of people employ guide dogs to assist in mobility. These dogs are trained to navigate around various obstacles, and to indicate when it becomes necessary to go up or down a step. However, the helpfulness of guide dogs is limited by the inability of dogs to understand complex directions. The human half of the guide dog team does the directing, based upon skills acquired through previous mobility training. In this sense, the handler might be likened to an aircraft's navigator, who must know how to get from one place to another, and the dog to the pilot, who gets them there safely. GPS devices can also be used as a mobility aid. Such software can assist blind people with orientation and navigation, but it is not a replacement for traditional mobility tools such as white canes and guide dogs. Some blind people are skilled at echolocating silent objects simply by producing mouth clicks and listening to the returning echoes. It has been shown that blind echolocation experts use what is normally the "visual" part of their brain to process the echoes.[48][49] Government actions are sometimes taken to make public places more accessible to blind people. Public transportation is freely available to the blind in many cities. Tactile paving and audible traffic signals can make it easier and safer for visually impaired pedestrians to cross streets. In addition to making rules about who can and cannot use a cane, some governments mandate the right-of-way be given to users of white canes or guide dogs. Reading and magnification Braille watch Most visually impaired people who are not totally blind read print, either of a regular size or enlarged by magnification devices. Many also read large-print, which is easier for them to read without such devices. A variety of magnifying glasses, some handheld, and some on desktops, can make reading easier for them. Others read Braille (or the infrequently used Moon type), or rely on talking books and readers or reading machines, which convert printed text to speech or Braille. They use computers with special hardware such as scanners and refreshable Braille displays as well as software written specifically for the blind, such as optical character recognition applications and screen readers. Some people access these materials through agencies for the blind, such as the National Library Service for the Blind and Physically Handicapped in the United States, the National Library for the Blind or the RNIB in the United Kingdom. Closed-circuit televisions, equipment that enlarges and contrasts textual items, are a more high-tech alternative to traditional magnification devices. There are also over 100 radio reading services throughout the world that provide people with vision impairments with readings from periodicals over the radio. The International Association of Audio Information Services provides links to all of these organizations. Computers and mobile technology Access technology such as screen readers, screen magnifiers and refreshable Braille displays enable the blind to use mainstream computer applications and mobile phones. The availability of assistive technology is increasing, accompanied by concerted efforts to ensure the accessibility of information technology to all potential users, including the blind. Later versions of Microsoft Windows include an Accessibility Wizard & Magnifier for those with partial vision, and Microsoft Narrator, a simple screen reader. Linux distributions (as live CDs) for the blind include Vinux and Adriane Knoppix, the latter developed in part by Adriane Knopper who has a visual impairment. macOS and iOS also come with a built-in screen reader called VoiceOver, while Google TalkBack is built in to most Android devices. The movement towards greater web accessibility is opening a far wider number of websites to adaptive technology, making the web a more inviting place for visually impaired surfers. Experimental approaches in sensory substitution are beginning to provide access to arbitrary live views from a camera. Modified visual output that includes large print and/or clear simple graphics can be of benefit to users with some residual vision.[50] Other aids and techniques A tactile feature on a Canadian banknote Blind people may use talking equipment such as thermometers, watches, clocks, scales, calculators, and compasses. They may also enlarge or mark dials on devices such as ovens and thermostats to make them usable. Other techniques used by blind people to assist them in daily activities include: Adaptations of coins and banknotes so that the value can be determined by touch. For example: In some currencies, such as the euro, the pound sterling and the Indian rupee, the size of a note increases with its value. On US coins, pennies and dimes, and nickels and quarters are similar in size. The larger denominations (dimes and quarters) have ridges along the sides (historically used to prevent the "shaving" of precious metals from the coins), which can now be used for identification. Some currencies' banknotes have a tactile feature to indicate denomination. For example, the Canadian currency tactile feature is a system of raised dots in one corner, based on Braille cells but not standard Braille.[51] It is also possible to fold notes in different ways to assist recognition. Labeling and tagging clothing and other personal items Placing different types of food at different positions on a dinner plate Marking controls of household appliances Most people, once they have been visually impaired for long enough, devise their own adaptive strategies in all areas of personal and professional management. For the blind, there are books in braille, audio-books, and text-to-speech computer programs, machines and e-book readers. Low vision people can make use of these tools as well as large-print reading materials and e-book readers that provide large font sizes. Computers are important tools of integration for the visually impaired person. They allow, using standard or specific programs, screen magnification and conversion of text into sound or touch (Braille line), and are useful for all levels of visual handicap. OCR scanners can, in conjunction with text-to-speech software, read the contents of books and documents aloud via computer. Vendors also build closed-circuit televisions that electronically magnify paper, and even change its contrast and color, for visually impaired users. For more information, consult Assistive technology. In adults with low vision there is no conclusive evidence supporting one form of reading aid over another.[52] In several studies stand-mounted devices allowed faster reading than hand-held or portable optical aids.[52] While electronic aids may allow faster reading for individuals with low vision, portability, ease of use, and affordability must be considered for people.[52] Children with low vision sometimes have reading delays, but do benefit from phonics-based beginning reading instruction methods. Engaging phonics instruction is multisensory, highly motivating, and hands-on. Typically students are first taught the most frequent sounds of the alphabet letters, especially the so-called short vowel sounds, then taught to blend sounds together with three-letter consonant-vowel-consonant words such as cat, red, sit, hot, sun. Hands-on (or kinesthetically appealing) VERY enlarged print materials such as those found in "The Big Collection of Phonics Flipbooks" by Lynn Gordon (Scholastic, 2010) are helpful for teaching word families and blending skills to beginning readers with low vision. Beginning reading instructional materials should focus primarily on the lower-case letters, not the capital letters (even though they are larger) because reading text requires familiarity (mostly) with lower-case letters. Phonics-based beginning reading should also be supplemented with phonemic awareness lessons, writing opportunities, and lots of read-alouds (literature read to children daily) to stimulate motivation, vocabulary development, concept development, and comprehension skill development. Many children with low vision can be successfully included in regular education environments. Parents may need to be vigilant to ensure that the school provides the teacher and students with appropriate low vision resources, for example technology in the classroom, classroom aide time, modified educational materials, and consultation assistance with low vision experts. Communication Communication with the visually impaired can be more difficult than communicating with someone who doesn't have vision loss. However, many people are uncomfortable with communicating with the blind, and this can cause communication barriers. One of the biggest obstacles in communicating with visually impaired individuals comes from face-to-face interactions.[53] There are many factors that can cause the sighted to become uncomfortable while communicating face to face. There are many non-verbal factors that hinder communication between the visually impaired and the sighted, more often than verbal factors do. These factors, which Rivka Bialistock[53] mentions in her article, include: Lack of facial expressions, mimics, or body gestures/responses Non-verbal gestures that could imply the visually impaired individual not appearing interested Speaking when not anticipated or not speaking when anticipated Fear of offending the visually impaired Standing too close and invading the personal comfort level Having to exercise or ignore feelings of pity Being uncomfortable with touching objects or people A look of detachment or disengagement Dependency Being reminded of the fear of becoming blind The blind person sends these signals or types of non-verbal communication without being aware that they are doing so. These factors can all affect the way an individual would feel about communicating with the visually impaired. This leaves the visually impaired feeling rejected and lonely. Adjusting attitude In the article Towards better communication, from the interest point of view. Or—skills of sight-glish for the blind and visually impaired, the author, Rivka Bialistock [53] comes up with a method to reduce individuals being uncomfortable with communicating with the visually impaired. This method is called blind-glish or sight-glish, which is a language for the blind, similar to English. For example, babies, who are not born and able to talk right away, communicate through sight-glish, simply seeing everything and communicating non-verbally. This comes naturally to sighted babies, and by teaching this same method to babies with a visual impairment can improve their ability to communicate better, from the very beginning. To avoid the rejected feeling of the visually impaired, people need to treat the blind the same way they would treat anyone else, rather than treating them like they have a disability, and need special attention. People may feel that it is improper to, for example, tell their blind child to look at them when they are speaking. However, this contributes to the sight-glish method.[53] It is important to disregard any mental fears or uncomfortable feelings people have while communicating (verbally and non-verbally) face-to-face. Surroundings Individuals with a visual disability not only have to find ways to communicate effectively with the people around them, but their environment as well. The blind or visually impaired rely largely on their other senses such as hearing, touch, and smell in order to understand their surroundings.[54] Sound Sound is one of the most important senses that the blind or visually impaired use in order to locate objects in their surroundings. A form of echolocation is used, similarly to that of a bat.[55] Echolocation from a person's perspective is when the person uses sound waves generated from speech or other forms of noise such as cane tapping, which reflect off of objects and bounce back at the person giving them a rough idea of where the object is. This does not mean they can depict details based on sound but rather where objects are in order to interact, or avoid them. Increases in atmospheric pressure and humidity increase a person's ability to use sound to their advantage as wind or any form of background noise impairs it.[54] Touch Touch is also an important aspect of how blind or visually impaired people perceive the world. Touch gives immense amount of information in the person's immediate surrounding. Feeling anything with detail gives off information on shape, size, texture, temperature, and many other qualities. Touch also helps with communication; braille is a form of communication in which people use their fingers to feel elevated bumps on a surface and can understand what is meant to be interpreted.[56] There are some issues and limitations with touch as not all objects are accessible to feel, which makes it difficult to perceive the actual object. Another limiting factor is that the learning process of identifying objects with touch is much slower than identifying objects with sight. This is due to the fact the object needs to be approached and carefully felt until a rough idea can be constructed in the brain.[54] Smell Certain smells can be associated with specific areas and help a person with vision problems to remember a familiar area. This way there is a better chance of recognizing an area's layout in order to navigate themselves through. The same can be said for people as well. Some people have their own special odor that a person with a more trained sense of smell can pick up. A person with an impairment of their vision can use this to recognize people within their vicinity without them saying a word.[54] Communication development Visual impairment can have profound effects on the development of infant and child communication. The language and social development of a child or infant can be very delayed by the inability to see the world around them. Social development Social development includes interactions with the people surrounding the infant in the beginning of its life. To a child with vision, a smile from a parent is the first symbol of recognition and communication, and is almost an instant factor of communication. For a visually impaired infant, recognition of a parent's voice will be noticed at approximately two months old, but a smile will only be evoked through touch between parent and baby. This primary form of communication is greatly delayed for the child and will prevent other forms of communication from developing. Social interactions are more complicated because subtle visual cues are missing and facial expressions from others are lost. Due to delays in a child's communication development, they may appear to be disinterested in social activity with peers, non-communicative and uneducated on how to communicate with other people. This may cause the child to be avoided by peers and consequently overprotected by family members. Language development With sight, much of what is learned by a child is learned through imitation of others, whereas a visually impaired child needs very planned instruction directed at the development of postponed imitation. A visually impaired infant may jabber and imitate words sooner than a sighted child, but may show delay when combining words to say themselves, the child may tend to initiate few questions and their use of adjectives is infrequent. Normally the child's sensory experiences are not readily coded into language and this may cause them to store phrases and sentences in their memory and repeat them out of context. The language of the blind child does not seem to mirror their developing knowledge of the world, but rather their knowledge of the language of others. A visually impaired child may also be hesitant to explore the world around them due to fear of the unknown and also may be discouraged from exploration by overprotective family members. Without concrete experiences, the child is not able to develop meaningful concepts or the language to describe or think about them.[57] Healthcare access Visual impairment has the ability to create consequences for health and well being. Visual impairment is increasing, especially among older people. It is recognized that those individuals with visual impairment are likely to have limited access to information and healthcare facilities, and may not receive the best care possible because not all health care professionals are aware of specific needs related to vision. Accommodation may require alternative means of communication.[58] Epidemiology The WHO estimates that in 2012 there were 285 million visually impaired people in the world, of which 246 million had low vision and 39 million were blind.[4] Of those who are blind 90% live in the developing world.[58] Worldwide for each blind person, an average of 3.4 people have low vision, with country and regional variation ranging from 2.4 to 5.5.[59] By age: Visual impairment is unequally distributed across age groups. More than 82% of all people who are blind are 50 years of age and older, although they represent only 19% of the world's population. Due to the expected number of years lived in blindness (blind years), childhood blindness remains a significant problem, with an estimated 1.4 million blind children below age 15. By gender: Available studies consistently indicate that in every region of the world, and at all ages, females have a significantly higher risk of being visually impaired than males. By geography: Visual impairment is not distributed uniformly throughout the world. More than 90% of the world's visually impaired live in developing countries.[59] Since the estimates of the 1990s, new data based on the 2002 global population show a reduction in the number of people who are blind or visually impaired, and those who are blind from the effects of infectious diseases, but an increase in the number of people who are blind from conditions related to longer life spans.[59] In 1987, it was estimated that 598,000 people in the United States met the legal definition of blindness.[60] Of this number, 58% were over the age of 65.[60] In 1994–1995, 1.3 million Americans reported legal blindness.[61] Society and culture See also: List of blind people and Blind musicians Legal definition To determine which people qualify for special assistance because of their visual disabilities, various governments have specific definitions for legal blindness.[62] In North America and most of Europe, legal blindness is defined as visual acuity (vision) of 20/200 (6/60) or less in the better eye with best correction possible. This means that a legally blind individual would have to stand 20 feet (6.1 m) from an object to see it—with corrective lenses—with the same degree of clarity as a normally sighted person could from 200 feet (61 m). In many areas, people with average acuity who nonetheless have a visual field of less than 20 degrees (the norm being 180 degrees) are also classified as being legally blind. Approximately fifteen percent of those deemed legally blind, by any measure, have no light or form perception. The rest have some vision, from light perception alone to relatively good acuity. Low vision is sometimes used to describe visual acuities from 20/70 to 20/200.[63] Literature and art See also: Blindness in literature Antiquity The Moche people of ancient Peru depicted the blind in their ceramics.[64] In Greek myth, Tiresias was a prophet famous for his clairvoyance. According to one myth, he was blinded by the gods as punishment for revealing their secrets, while another holds that he was blinded as punishment after he saw Athena naked while she was bathing. In the Odyssey, the one-eyed Cyclops Polyphemus captures Odysseus, who blinds Polyphemus to escape. In Norse mythology, Loki tricks the blind god Höðr into killing his brother Baldr, the god of happiness. The New Testament contains numerous instances of Jesus performing miracles to heal the blind. According to the Gospels, Jesus healed the two blind men of Galilee, the blind man of Bethsaida, the blind man of Jericho and the man who was born blind. The parable of the blind men and an elephant has crossed between many religious traditions and is part of Jain, Buddhist, Sufi and Hindu lore. In various versions of the tale, a group of blind men (or men in the dark) touch an elephant to learn what it is like. Each one feels a different part, but only one part, such as the side or the tusk. They then compare notes and learn that they are in complete disagreement. "Three Blind Mice" is a medieval English nursery rhyme about three blind mice whose tails are cut off after chasing the farmer's wife. The work is explicitly incongruous, ending with the comment Did you ever see such a sight in your life, As three blind mice? Modern times Blind Woman by Diego Velázquez The Sense of Touch by Jusepe de Ribera depicts a blind man holding a marble head in his hands. Poet John Milton, who went blind in mid-life, composed On His Blindness, a sonnet about coping with blindness. The work posits that [those] who best Bear [God]'s mild yoke, they serve him best. The Dutch painter and engraver Rembrandt often depicted scenes from the apocryphal Book of Tobit, which tells the story of a blind patriarch who is healed by his son, Tobias, with the help of the archangel Raphael.[65] Slaver-turned-abolitionist John Newton composed the hymn Amazing Grace about a wretch who "once was lost, but now am found, Was blind, but now I see." Blindness, in this sense, is used both metaphorically (to refer to someone who was ignorant but later became knowledgeable) and literally, as a reference to those healed in the Bible. In the later years of his life, Newton himself would go blind. H. G. Wells' story "The Country of the Blind" explores what would happen if a sighted man found himself trapped in a country of blind people to emphasise society's attitude to blind people by turning the situation on its head. Bob Dylan's anti-war song "Blowin' in the Wind" twice alludes to metaphorical blindness: How many times can a man turn his head // and pretend that he just doesn't see... How many times must a man look up // Before he can see the sky? Contemporary fiction contains numerous well-known blind characters. Some of these characters can see by means of devices, such as the Marvel Comics superhero Daredevil, who can see via his super-human hearing acuity, or Star Trek's Geordi La Forge, who can see with the aid of a VISOR, a fictional device that transmits optical signals to his brain. Sports Blind and partially sighted people participate in sports, such as swimming, snow skiing and athletics. Some sports have been invented or adapted for the blind, such as goalball, association football, cricket, golf, tennis, bowling, and beep baseball.[66][67] The worldwide authority on sports for the blind is the International Blind Sports Federation.[68][69] People with vision impairments have participated in the Paralympic Games since the 1976 Toronto summer Paralympics.[70] Metaphorical uses The word "blind" (adjective and verb) is often used to signify a lack of knowledge of something. For example, a blind date is a date in which the people involved have not previously met; a blind experiment is one in which information is kept from either the experimenter or the participant to mitigate the placebo effect or observer bias. The expression "blind leading the blind" refers to incapable people leading other incapable people. Being blind to something means not understanding or being aware of it. A "blind spot" is an area where someone cannot see: for example, where a car driver cannot see because parts of his car's bodywork are in the way; metaphorically, a topic on which an individual is unaware of their own biases, and therefore of the resulting distortions of their own judgements (see Bias blind spot). Research Main article: Visual prosthesis A 2008 study tested the effect of using gene therapy to help restore the sight of patients with a rare form of inherited blindness, known as Leber's congenital amaurosis or LCA.[71] Leber's Congenital Amaurosis damages the light receptors in the retina and usually begins affecting sight in early childhood, with worsening vision until complete blindness around the age of 30. The study used a common cold virus to deliver a normal version of the gene called RPE65 directly into the eyes of affected patients. Remarkably, all 3 patients, aged 19, 22 and 25, responded well to the treatment and reported improved vision following the procedure. Due to the age of the patients and the degenerative nature of LCA, the improvement of vision in gene therapy patients is encouraging for researchers. It is hoped that gene therapy may be even more effective in younger LCA patients who have experienced limited vision loss, as well as in other blind or partially blind individuals. Two experimental treatments for retinal problems include a cybernetic replacement and transplant of fetal retinal cells.[72] Other animals Main article: Blindness in animals Statements that certain species of mammals are "born blind" refers to them being born with their eyes closed and their eyelids fused together; the eyes open later. One example is the rabbit. In humans, the eyelids are fused for a while before birth, but open again before the normal birth time; however, very premature babies are sometimes born with their eyes fused shut, and opening later. Other animals, such as the blind mole rat, are truly blind and rely on other senses.[citation needed] The theme of blind animals has been a powerful one in literature. Peter Shaffer's Tony Award-winning play, Equus, tells the story of a boy who blinds six horses. Theodore Taylor's classic young adult novel, The Trouble With Tuck, is about a teenage girl, Helen, who trains her blind dog to follow and trust a seeing-eye dog. See also Acute visual loss Blindness and education Color blindness Diplopia Nyctalopia Recovery from blindness Stereoblindness Tactile alphabet Tactile graphic Tangible symbol systems Visual agnosia Visual impairment due to intracranial pressure World Blind Union References "Change the Definition of Blindness" (PDF). World Health Organization. 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"Bionic Eye Opens New World Of Sight For Blind". NPR. Retrieved 9 March 2019. External links Wikimedia Commons has media related to Visual disturbances and blindness. Blindness at Curlie Chisholm, Hugh, ed. (1911). "Blindness" . Encyclopædia Britannica (11th ed.). Cambridge University Press -
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