Intensity of Electromagnetic Waves on Spherical Surface

The intensity $I$ of electromagnetic waves emitted isotropically by a point light source in free space at a distance $r$ from the source is given by the inverse-square law:

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

where $P$ is the total power emitted by the point source.

At the initial position, the detector is placed at a distance $r_1 = L\text{ m}$ from the source, so the measured intensity is:

$I_0 = \frac{P}{4\pi L^2}$

When the detector is shifted to another location on the same spherical surface, the distance of the detector from the point source remains equal to the radius of the sphere:

$r_2 = L\text{ m}$

Since the intensity depends only on the radial distance $r$ from the point source, the intensity at the new location is:

$I' = \frac{P}{4\pi r_2^2} = \frac{P}{4\pi L^2} = I_0$

Thus, the measured intensity at the new location remains $I_0$.

Correct Option: B