Q. A point source emits sound equally in all directions in a non-absorbing medium. Two points P and Q are at distance of 2m and 3m respectively from the source. The ratio of the intensities of the waves at P and Q is
A) 9:4
B) 2:3
C) 3:2
D) 4:9
Explanation
(a) The average power per unit area that is incident perpendicular to the direction of propagation is called the intensity
Intensity of sound
$I = \frac{P}{4\pi r^{2}}$
Or
$I \propto \frac{1}{r^{2}}$
Or
$\frac{I_{1}}{I_{2}} = \left( \frac{r_{2}}{r_{1}} \right)^{2}$
Here, $r_{1} = 2m,\ r_{2} = 3m$
Substituting the values, we have
$\frac{I_{1}}{I_{2}} = \left( \frac{3}{2} \right)^{2} = \frac{9}{4}$
Reviewed by Dr. Abubacker
Physics Educator & NEET/JEE Specialist
Ph.D. in Physics with 19+ years of teaching experience.