Question

As a wave propagates,

• A. the wave intensity remains constant for a planewave
• 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 form the source for a spherical wave
• D. the wave intensity decreases as the inverse of the distance for a line source

Answer 1

Answer: (A), (C), (D)

the wave intensity decreases as the inverse square of the distance form the source for a spherical wave
the wave intensity remains constant for a planewave
the wave intensity decreases as the inverse of the distance for a line source

A plane wave does not have any radial dependence. Thus a plane wave travels with constant intensity. Intensity of a spherical wave decreases. It is because the power in the wave is carried and constant at all radius. Intensity is power distributed over the spherical surface area.

$I=\frac{P}{4\pi r^2}$

For a cylindrical wave the power is same at all distance from the source. The power is distributed over the entire surface area.
$I=\frac{P}{2\pi rl}$