Such explanations do not take into account other environmental factors, such as a bad diet or poor. Causes:

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$Myopia (nearsightedness) Myopia is a name used to describe the refractive disorder known as nearsightedness.$

1 Myopia (nearsightedness) Myopia is a name used to describe the refractive disorder known as nearsightedness. With nearsightedness, light rays from nearby objects focus on the retina, but distant objects focus in front of the retina, causing those distant objects to be blurred. This generally occurs because the eyeball is larger than normal, so light rays must travel a longer distance to the retina. This longer distance causes light which would normally focus on the retina to fall a little bit short and instead focus in front of the retina. This focusing of light in front of the retina can also occur if the cornea and/or lens is more sharply curved than normal. Whichever the case, the eyes' refractive - or dioptric - power (how much it "bends" light rays") is said to be too strong. Some scientists believe this disorder to be inherited. Babies are not usually born nearsighted. Generally it first occurs in kids 5-8 years old. When the child becomes a teenager and grows rapidly, the myopia often worsens. Fortunately for most, this usually levels off when they reach their 20's. This debate over whether myopia is inherited or acquired has been going on for some time now. Experiments have shown a trend in which environmental conditions, such as people who attended school as opposed to those who didn't, appear to have an impact on the development of myopia in a person. One explanation supporting the acquired theory goes like this: The ciliary muscle (the ring-shaped muscle behind the iris) is relaxed when you rest your eyes or focus on something far away. The ciliary contracts when focusing on things up close, making the lens more convex. For some people, when the ciliary is tens ed like this for a long time, for example during long periods of reading, the muscle spasms. It gets more difficult for it to relax as needed to focus on things at a distance. The spasming increases the pressure of the liquid which keeps the eyeball in shape. This increase in pressure in turn pushes the front portion of the eye outward. Since the distance from lens to retina has increased, the focus of the light rays falls short, resulting in a blurred image. Another explanation for the development of myopia in children suggests a progressive myopia concept as follows: Prescribed distance glasses for light myopic kids, intended to be used when viewing things at distances, are also used by these kids when reading when not necessary. This aggravates the problem, since the eye must work even harder to focus on things up close, forcing the eye to become more myopic. This is because glasses for nearsightedness spread out (diverges) light rays rather than focus them. When viewing things far away, this is good since a myopic eye's focusing power is too strong. However, a light myopic person may be able to see things up close fine without the glasses. With glasses though, the eye must work harder to focus those diverging rays. Essentially, wearing the right glasses at the wrong time is bad for your eyes. Such explanations do not take into account other environmental factors, such as a bad diet or poor

2 lighting. In any case, it appears that myopia occurs because of both genetic (inherited) and environmental factors. Doctors prescribe eyeglasses or contacts with concave lenses (thin in the middle but thick at the edges) to help focus distant rays on the retina. There are also a number of (laser) surgery options available to restore vision partially or even fully for some. Hyperopia (farsightedness) People with hyperopia tend to see distant objects clearly or more clearly than objects up close, which are blurry. This is because the light rays don't get a chance to come into focus before they reach the retina. Hyperopia is a refractive disorder. Generally this disorder occurs because there is less distance between the cornea and retina than in a normal eye. This shorter eyeball therefore causes most light rays, especially those from nearby objects, to reach the retina before coming into focus. The hyperopic eyes' refractive power is too weak. When viewing objects far away, the lens of a normal eye is pretty flat. However, the lens of a farsighted eye cannot accommodate, or change its shape, enough to make the near vision clear. Often, the lens must accommodate even when seeing at far distances. This extra work that a hyperopic eye must do can cause eyestrain and headaches. Eyeglasses or contacts with convex lenses (thick in the middle and thin at the edges) are prescribed to help focus nearby light rays on the retina. There are also some laser surgery options available to restore farsighted vision partially, or even fully in some cases. Presbyopia As people age, the lens lose the ability to accommodate as well as they could when they were younger. This condition, presbyopia, begins affecting people in their forties or fifties, causing people to lose their clear near vision. This condition is another refractive disorder. Age is usually the cause of this disorder. The lens becomes less flexible with age, and therefore encounters more and more difficulty trying to focus on objects up close. The hardening lenses of the eyes causes vision up close to be blurrier and blurrier. The average 10 year old can still focus on objects

3 just 3 inches away from the eyes. This closes point at which an object can be viewed in focus is called the near point of accommodation. At age 12, this point has decreased to about 4 inches. By age 40, the distance has increased up to 6 to 10 inches. This increases to around 39 inches by the time a person reaches 60 years old. This is why most old people can't read without glasses. Glasses or contacts are commonly used to correct presbyopia. People who needed glasses when younger now may need 2 pairs, one used for seeing things far away, and the other to see things up close. Oftentimes people use bifocals instead of two pairs of glasses, with the top portion used for distance viewing and the lower for reading. Bifocal contacts have a central circle for distance viewing, while the surrounding area contains the prescription for reading. Astigmatism A person with astigmatism has distorted vision at any distance. This is because the cornea and/or lens are irregularly shaped. This uneven curvature scatters some of the incoming light rays in such a way that some focus in front of the retina, on the retina (like a non-astigmatic eye should), and behind the retina. Eye injury or a disease that scars the cornea may cause astigmatism. Some people are simply born with it. Contacts or glasses with cylindrical lenses usually are used to correct astigmatism. Since light rays hit the eyes from many directions, the cylindrical lenses bend some of the light rays coming from a particular direction more than the others, focusing the light on the retina. Contacts may clear ip astigmatism better than glasses since the light rays passing through the smoothly shaped contacts act as if they passed through a smooth cornea. However, contacts may be uncomfortable for those with heavy corneal astigmatism. Severe corneal astigmatisms may need corneal transplants. This operation removes the scarred cornea and replaces it with a smooth, clear cornea from a donor. There are also some laser surgery options that may correct astigmatism. Strabismus - e.g. cross-eye and more The term strabismus is used to describe the condition in which the eyes do not move together in a coordinated fashion. This is usually because one of the eyes' muscles are weak. Cross-eye (a.k.a. esotropia), the condition where one eye looks straight ahead while the other moves towards the nose, is just one particular form of strabismus. Other forms include exotropia, hypertropia, and hypotropia.

4 With exotropia (a.k.a. wall-eye), one eye wanders outwards to the side of the head. People with hypertropia have one eye turned upwards. Hypotropic people have one eye that turns downward. If this condition is not treated, strabismus may also lead to amblyopia, or "lazy eye", another condition in which the child loses some or all vision from the weak eye. Strabismus can be inherited, caused by an injury, or caused by illness. It commonly shows up in severely farsighted young children. It may also occur because the vision through one eye is much worse than the other. A patch may be placed over the stronger eye, forcing the weak eye to do all the seeing for a certain period of time. This is to strengthen the weaker eye, so that the eyes work together better. Another way to force the weak eye to be used for seeing involves using eye drops which blur the vision in the good eye. Glasses may also be prescribed. In other cases, an eye doctor may recommend surgery in conjunction with eye exercises to coordinate the patient's eye movements. Amblyopia (lazy eye) Also commonly referred to as "lazy eye", this term describes the condition in which the brain suppresses the image coming from the weaker eye. Strabismus may lead to amblyopia, causing the weak eye to encounter difficulty developing. A large difference in the vision of one eye compared to the other may also cause amblyopia. If amblyopia arises because one eye has significantly worse vision than the other, glasses or contacts may be used to correct the problem. In other cases, the weak eye needs strengthening, as in the case with strabismus. Eye patches, eye drops, or even surgery may be used as corrective methods as well. Color Blindness (dichromasy) Recall that the human eye is sensitive to the three primary colors of light - red, blue and green - and that this is possible because the eye has one type of cone sensitive to each of these colors. If a person can't see one of these basic colors, then he or she won't be able to see the colors created from combining that color with the other two basic colors. Instead, the person will most likely confuse those shades with others they can see.

5 Dichromasy, the major type of colorblindness, is a term describing people who have only two of the three major types of cones in their eyes. The three types of dichromasy are: Protanopia Deuteranopia Tritanopia Red blindness - Protanopes cannot see reds, or any color that is a mix of red and the other two primary colors. These shades of red are confused with shades of green or blue. For example: red violet blue A mix of red and blue (violet) would appear blue to a protanope. Green blindness - Deutanopes cannot see greens, or any color that is a mix of green and the other two primary colors. These shades of green are usually confused with shades of red. For example: green white violet A mix of green and violet (white) would appear purplish to a deutanope. Blue blindness (rare) - Tritanopes cannot see blues, or any color that is a mix of blue and the other two primary colors. These shades of blue are confused with shades of red or green. For example: blue cyan green A mix of blue and green (cyan) would appear greenish to a tritanope. Both protanopes and deutanopes are considered red-green blind, since they usually confuse reds with greens and vice-versa. Those who are completely color blind have achromatic vision, meaning they can't see any colors at all. To them, there would be no difference between seeing something in real life and the same image on a black and white television. In almost all cases, color blindness is genetic - an inherited disorder. A boy is much more likely to inherit it than a girl from their parents. This is because color blindness is a sex-linked trait. This means that a boy only needs to inherit one gene from his mother or father to be colorblind, while a girl must inherit two genes - one from her mom and one from her dad - to be color blind. If a girl has only one color blindness gene, then she is considered a carrier. That means that she can pass it on to her children, but she herself does not exhibit color blindness. It is currently not possible to "cure" color blindness. However, special contacts have been developed that help color blind people tell apart certain colors they wouldn't normally be able to distinguish. Also, for example, color blind people could drive since the traffic lights are always in a standard position. Detection: A variety of tests are used to detect color blindness and the degree of it. You may be familiar with the tests which require the viewer to identify shapes or numbers in a bunch of colored dots. People with normal vision would see the object, while a color blind person would see a different object, or no distinguishable object at all.

PHYS:1200 LECTURE 31 LIGHT AND OPTICS (3)

1 PHYS:1200 LECTURE 31 LIGHT AND OPTICS (3) In lecture 30, we applied the law of reflection to understand how images are formed using plane and curved mirrors. In this lecture we will use the law of refraction