Question

From a point source, if amplitude of water at a distance 'r' is A, its amplitude at a distance '2r' will be

• A. A
• B. 2A
• C. A/2
• D. A/4

Answer 1

The correct option is C A/2

Intensity = energy/sec/area = power/area
From a point source, energy spreads over the surface of a sphere of radius r.
I=P/A=P/(4$\pi r^2$)
But intensity $I=(\text{amplitude}{)}^2$
$(\text{amplitude}{)}^2\propto \frac{1}{r^2}$
At a distance 2r, amplitude becomes A/2.
Ans: C