Q.Mention the defects of human vision and describe the method to remove them.

Myopia → concave lens; hypermetropia → convex lens; presbyopia → bifocal lens; astigmatism → cylindrical lens.

1. Myopia (short-sightedness). The eye can see nearby objects clearly but distant objects appear blurred. It occurs because the eyeball is too long or the eye lens is too converging, so the image of a distant object forms in front of the retina. The far point comes closer than infinity.

Correction: use spectacles with a concave (diverging) lens of suitable focal length, which diverges the incoming parallel rays so that the image is pushed back onto the retina. The lens focal length equals (negative of) the defective far-point distance.

2. Hypermetropia (long-sightedness / far-sightedness). The eye can see distant objects clearly but nearby objects appear blurred. It occurs because the eyeball is too short or the eye lens is too weak, so the image of a nearby object forms behind the retina. The near point moves farther than 25 cm.

Correction: use spectacles with a convex (converging) lens, which converges the rays before they enter the eye so the image is brought forward onto the retina.

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