Question

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

Answer 1

Hint: Myopia is caused by a slightly elongated eyeball in which the lens reflects light in front of the retina rather than directly on it. While glasses, contact lenses, eye drops, and surgery can correct the effects of myopia and enable good distance vision, they only treat the symptoms of the disease.

Complete answer:
We know that the object that is the moon is at infinity distance. Hence,
$u = - \infty $
We also know that if the object is at infinity then the image is formed at focal length. Now,
$f = - 100\,cm$
Myopic is corrected by concave lenses in eyeglasses. Near-sighted individuals can't see distant points clearly because the gap between the lens and the retina of their eyes is longer than it should be. As concave lenses are placed in front of a near-sighted eye, the refraction of light is reduced and the focal length is lengthened, allowing the vision to be created on the retina.

Hence, the concave lens will enable him to see the moon distinctly if its focal length is $100\,cm$.

Note:Myopia is a vision disorder in which optical signals focus in front of the lens of the eye, causing blurry vision of distant objects. Myopic has a broader definition. Someone who is myopic can have difficulty viewing it from multiple perspectives or understanding the long-term implications of their actions before taking action.