Inverse square law :

An object by a light source is inversely proportional to square of the distance from the source.

In the dark room the person is trying to read a book.

Consider the distance between the person and light source is .

From the inverse square law .

Light is too dim, so the person moves to half distance to the lamp.

Then the distance between the person and the light source is .

From the inverse square law,

Find brightness of the light if he moves to half distance to the lamp.

Ratio between  and  gives the brightness of the light if he moves to half distance to the lamp.

The light is  times brighter when the person moves to half distance to the lamp.

The light is  times brighter when the person moves to half distance to the lamp.