Question

The inverse square law of intensity (i.e. the intensity $\infty \frac{1}{r^2}$) is valid for a
(a) point source
(b) line source
(c) plane source
(d) cylindrical source

Answer 1

(a) point source

Intensity of a point source obeys the inverse square law.
Intensity of light at distance r from the point source is given by
$I=S/\left(4{\mathrm{\pi r}}^2\right)$ ,
where S is the source strength.