Q 12-08-039JEE MainJEE Main 2026 (24 Jan, Shift 2)Easy
A point source is kept at the center of a spherically enclosed detector. If the volume of the detector increased by 8 times, the intensity will
Answer: (B) decrease by 4 times
$V\propto r^3$: increasing the volume 8 times doubles the radius.
For a point source the power spreads over the sphere: $I = \dfrac{P}{4\pi r^2}\propto\dfrac1{r^2}$. Doubling $r$ makes the intensity $\dfrac14$ of its value, i.e. it decreases by 4 times.
Solution by Sreeraj P, M.Sc Physics