Question

A person wearing -1.5 D eyeglasses 2.5 cm in front of her eyes is looking at the night sky. If her retina is 2.0 cm behind her eye lens, what should the focal length of the eye lens be for her to see the stars clearly

Answer 1

Step-by-step explanation:

Power of a lens is the reciprocal of its focal length. It is measured in dioptres (D). Mathematically;

`P = (1)/(f)`

f is the focal length of the lens

Given

Power of the lens = -1.5D

Substitute into the formula;

`f = (1)/(D) \\f = (1)/(-1.5)\\f = -0.67 cm`

Hence the focal length of the eye lens for her to see clearly should be 0.67cm (concave lens)