In this post we would learn about errors of refraction.
Astigmatism: - Defective curvature of the cornea or lens of the eye is called Astigmatism. From one or more abnormal curvatures of the cornea or lens this problem leads in. This causes light rays to be unevenly and not sharply focused on the retina, so that the image is distorted. A cylindrical lens placed in the proper position in front of the eye can correct this problem.
Hyperopia: - The eye ball in this condition is too short or the refractive power of the lens is too weak. Parallel rays of light tend to focus behind the retina and this result in a blurred image. A convex lens which is thicker in the middle than at tht sides bends the rays inward before they reach the cornea, and thus the rays can be focused properly on the retina.
Myopia: - In myopia the eyeball is too long or the refractive power of the lens so strong that light rays does not properly focus on the retina. The image perceived is blurred because the light rays are focused in front of the retina. Concave glasses which are thicker at the periphery than in the middle correct this condition because the lenses spread the rays out before they reach the cornea, and thus they can be properly focused directly on the retina.
Presbyopia: - This is the impairment of vision due to old age. With increasing age, loss of elasticity of the ciliary body impairs its ability to adjust the lens for accommodation to near vision. The lens of the eye cannot become fat to bend the rays coming from near objects are less than 20 feet. The light rays focus behind the retina, as in hyperopia. Therefore, a convex lens is needed to refract the rays coming from objects closer than 20 feet.
In the next post we will learn about pathological conditions of refraction.
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