Question

The intensity at a point at a distance $r$ from a source which produces cylindrical wavefronts varies as

• A. $I \propto \frac{1}{r^{2}}$
• B. $I \propto \frac{1}{r}$
• C. $I \propto r^{0}$
• D. $I \propto \frac{1}{r^{3}}$

Answer 1

Answer: (B)

The intensity at a point at a distance $r$ from a source varies as $I \propto \frac{1}{r}$

This is because intensity is $\frac{\text { Power }}{\text { Area }}$ and the surface area of wavefronts changes by $2 \pi r l$.