Q.A person suffering from myopia cannot see distant objects clearly and has a far point of only $2\ \text{m}$. Find the power and nature of the spectacle lens that will let him see very distant objects clearly.

In myopia the eye's image of a distant object forms before the retina (due to an elongated eyeball or excessive curvature of the lens); a concave lens diverges the incoming rays just enough to shift the image back onto the retina.

Myopia (short-sightedness/near-sightedness)

Myopia is a common defect of vision in which a person can see nearby objects clearly, but distant objects appear blurred.

Cause: This happens because the image of a distant object is formed in front of the retina, not on it — either because (i) the eyeball has become too elongated (increased length), or (ii) the eye lens's focal length is too short (its converging power is too great), so light from a distant object converges before reaching the retina.

A myopic eye also has a far point that is nearer than infinity — i.e., it cannot see objects placed at very large distances clearly.

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Hypermetropia is far-sightedness caused by a shortened eyeball or reduced converging power of the eye lens; a convex lens corrects it by converging the light a little before it enters the eye.

What is hypermetropia?

Hypermetropia, also called long-sightedness or far-sightedness, is a common eye defect in which a person can see distant objects clearly but cannot see nearby objects distinctly. It occurs because:

• the eyeball has become too small (axially too short), or
• the focal length of the eye lens is too long (the ciliary muscles cannot increase the eye lens's curvature/power enough).

As a result, the image of a nearby object is formed behind the retina instead of exactly on it, producing a blurred image. The near point of the eye shifts farther away from the eye than the normal near point of 25 cm.

Correction: …