Question

As the wave propagates, choose the incorrect statement.

• A. the wave intensity remains constant for a plane curve
• B. the wave intensity decreases as the inverse of the distance from the source for a spherical wave
• C. the wave intensity decreases as the inverse square of the distance from the source for a spherical wave
• D. the wave intensity decreases as the inverses of the distance for a line source

Answer 1

Answer: (B)

For a plane wave intensity (energy crossing per unit area per unit time) is constant at all points.

But for a spherical wave, intensity at a distance $r$ from a point source of power $P$ (energy transmitted per unit time) is given by
$I=\frac{P}{4\pi r^2}$ or $I\propto \frac{1}{r^2}$
For a line source
$I=\frac{P}{2\pi r\ell }$ or $I\propto \frac{1}{r}$