Question

The intensity produced by a long cylindrical light source at a small distance $r$ from the source is proportional to

• A. $\frac{1}{r^2}$
• B. $\frac{1}{r^3}$
• C. $\frac{1}{r}$
• D. None of these

Answer 1

Answer: (C)

$\frac{1}{r}$

At a distance $r$ from a line source of power$P$ and length $l$, the intensity will be

$I=\frac{P}{S}=\frac{P}{2\pi rl}\Rightarrow I\propto \frac{1}{r}$