Q.Mention the defects of human vision and describe the method to remove them.
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Start your 14-day free trial to unlock the full solution →The eye's common defects — myopia (concave lens), hypermetropia (convex lens), presbyopia (bifocal lens) and astigmatism (cylindrical lens) — arise from focusing errors and are corrected with suitable spectacle lenses.
A normal eye focuses light from objects between infinity (far point) and about 25 cm (near point) sharply on the retina. When this fails, the eye has a defect of vision. The main defects and their remedies are:
1. Myopia (short-sightedness / near-sightedness).
Defect: the person can see nearby objects clearly but distant objects appear blurred. Parallel rays from a far object are converged in front of the retina, because either the eyeball is too long or the eye lens is too converging; the far point shifts nearer than infinity.
Correction: use spectacles with a concave (diverging) lens of suitable focal length, which diverges the incoming rays slightly so the image forms exactly on the retina.
2. Hypermetropia (long-sightedness / far-sightedness).
Defect: the person can see distant objects clearly but near objects appear blurred. Rays from a nearby object are converged behind the retina, because the eyeball is too short or the lens too weak; the near point shifts farther than 25 cm.
Correction: use spectacles with a convex (converging) lens, which adds the needed converging power so the near-object image falls on the retina.
3. Presbyopia. …
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