Question

a person is not able to read from a book until he keeps it at 50 cm determine the power of corrective lens

Answer 1

Since the person is able to read clearly from a distance of 50 cm,

focal length required = 50 cm = 0.5m

Power = \frac{1}{f}= \frac{1}{0.5}=2D

Power of corrective lens is 2D.

Answer 2

the person is suffering from hypermetropia. the power of corrective lens will be in negative and he will be using concave lens