If you are reading a book and you move twice as far away from the light source, how does the brightness at the new distance compare with that at the old distance? It is

The intensity of light is directly proportional to its brightness and by the inverse square law, intensity is inversely proportional to the square of the distance from the light source. Hence if you move twice as far away from the light source, the brightness at the new distance is 4 times less than the brightness at the old distance.