Defects in Human Eye

8/13/2019 Defects in Human Eye

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DEFECTS OF HUM A N EYE VISION.


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INTRODUCTION

Eye, light-sensitive organ of vision in animals. The

eyes of various species vary from simple structures that arecapable only of differentiating between light and dark tocomplex organs such as those of humans and othermammals that can distinguish minute variations of shape,

color, brightness, and distance. The actual process of seeing isperformed by the brain rather than by the eye. The function ofthe eye is to translate the electromagnetic vibrations of lightinto patterns of nerve impulses that are transmitted to thebrain.


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Astigmatism

This defect is when the light rays

do not all come to a single focal point on theretina, instead some focus on the retina andsome focus in front of or behind it. This isusually caused by a non-uniform curvature of

the cornea. A typical symptom of astigmatismis if you are looking at a pattern of linesplaced at various angles and the lines runningin one direction appear sharp whilst those inother directions appear blurred. Astigmatismcan usually be corrected by using a specialspherical cylindrical lens; this is placed in theout-of-focus axis.


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DIFFERENT FOCUS OF EYE

Focusing the Eye

Light rays entering the eye are refracted, or bent, when they passthrough the lens. Normal vision requires that the rays focus on the retina. If theeyeball is too long, an accurately focused image falls short of the retina. This iscalled myopia, or shortsightedness. A shortsighted person sees distant objectsunclearly. Longsighted focus, or hyperopia, results when the eyeball is too short.

In this case, an accurately focused image would fall behind the retina.


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A concave lens is curved inward; it is shaped like twodishes placed back-to-back. Light passing through a concave lensbends outward, or diverges. Unlike convex lenses, which producereal images, concave lenses produce only virtual images. A virtualimage is one from which light rays only appear to come. This one

appears as a smaller image just in front of the actual object (in thiscase a shamrock). Concave lenses are generally prescribed formyopic, or short-sighted, people. Concave lenses help the eyes toproduce a sharp image on the retina instead of in front of it.

CONCAVE LENS


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A convex lens has a thick centre and thinner edges. Light passingthrough a convex lens is bent inward, or made to converge. Thiscauses an image of the object to form on a screen on the oppositeside of the lens. The image is in focus if the screen is placed at aparticular distance from the lens that depends upon the distance

of the object and the focal point of the lens. The lens in thehuman eye is convex, but, unlike a glass lens, it is elastic so that itcan change shape to focus on objects at varying distances. Thelens becomes short and fat when viewing close objects and

elongated and thin when viewing distant objects.

CONVEX LENS


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This scanning electron microscope (SEM) is to the left of

the operator, with the computer images of the specimen on thescreens to the right. Although the SEM cannot see objects as smallas those that a transmission electron microscope can resolve, theimages it produces are more useful for seeing the three-dimensionalsurface structure of small objects .

SCANNING ELECTRON MICROSCOPE


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MAGNIFYING GLASS

A magnifying glass is a large convex lens commonly used toexamine small objects. The lens bends incoming light virtual so

that an enlarged , image of the object (in this case a mushroom)appears beyond it. The image is called virtual because the lightrays that appear to come from it do not actually pass through it.


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Defects in Human Eye