HUMAN EYE AND LENSES

INTRODUCTION Eye is the light-sensitive organ of vision in animals. The actual process of seeing is performed by the brain rather than by the eye. The function of the eye is to translate the electromagnetic vibrations of light into patterns of nerve impulses that are transmitted to the brain.

DEFECTS OF THE EYE Myopia: (nearsightedness) This is a defect of vision in which far the refractive power of the eye’s lens too strong. objects appear blurred but near objects are seen clearly. The image is focused in front of the retina rather than on it usually because the eyeball is too long or Myopia can be corrected by wearing glasses/contacts with concave lenses these help to focus the image on the retina

Hyperopia: (farsightedness) This is a defect of vision in which there is difficulty with near vision but far objects can be seen easily. The image is focused behind the retina rather than upon it. This occurs when the eyeball is too short or the refractive power of the lens is too weak. Hyperopia can be corrected by wearing glasses/contacts that contain convex lenses.

Astigmatism: This defect is when the light rays do not all come to a single focal point on the retina, instead some focus on the retina and some focus in front of or behind it. This is usually caused by a non-uniform curvature of the cornea. A typical symptom of astigmatism is if you are looking at a pattern of lines placed at various angles and the lines running in one direction appear sharp whilst those in other directions appear blurred. Astigmatism can usually be corrected by using a special spherical cylindrical lens; this is placed in the out-of-focus axis.

Focusing the Eye Light rays entering the eye are refracted, or bent, when they pass through the lens. Normal vision requires that the rays focus on the retina. If the eyeball is too long, an accurately focused image falls short of the retina.

A concave lens is curved inward. Light passing through a concave lens bends outward, or diverges. Concave lenses produce virtual images, as a smaller image just in front of the actual object (in this case a leaf). Concave lenses are generally prescribed for myopic, or short-sighted, people. CONCAVE LENS

A convex lens has a thick centre and thinner edges. Light passing through a convex lens is bent inward, or made to converge. This causes an image of the object to form on a screen on the opposite side of the lens. The lens in the human eye is convex, it is elastic so that it can change shape to focus on objects at varying distances. The lens becomes short and fat when viewing close objects and elongated and thin when viewing distant objects. CONVEX LENS

. MAGNIFYING GLASS A magnifying glass is a large convex lens commonly used to examine 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 light rays that appear to come from it do not actually pass through it.

Mrs. SAGAYA RANI.S; St. Joseph’s Hr. Sec. School; Chennai