A good photographic print is obtained by an exposure of two seconds at a distance of `20` cm from the lamp. Calculate the time of exposure required to get an equally good result at a distance of `40` cm.

Explanation

We know that the intensity of light varies inversely as the `("distance")^(2)` . When distance is double the intensity becomes one- fourth . So time of exposure should be four times.
Hence time of exposure `=2xx 4 =8 s`