Question

A myopic person can see things clearly if they lie between $8cm$ and $100cm$ from his eye. The lens will enable him to see the moon distinctly if its focal length is:

Answer 1

Concave lens:

1. Concave lens is a lens that is thin at the middle and thick at the edges.
2. It is a diverging lens. It diverges the light rays falling on it.

Myopia:

1. Myopia or shortsightedness is a condition in which a person can clearly see the objects near to them and the objects that are distant appear blurry.
2. The myopic eye converges rays from distant objects in front of the retina.
3. A person with this condition is a myopic person.

Correction of myopia:

1. Myopia can be corrected by using a concave lens or lenses that are of negative focal length.
2. A concave lens can correct myopia as it diverges the light rays that enter the eye such that a little further back directly on the retina.
3. The final lens image of the moon must be formed at a distance of $100cm$, so the focal length of the concave lens should be $-100cm$.

Hence, the focal length should be $-100cm$.