The intensity of an electromagnetic (EM) wave typically decreases with distance from the source following the inverse square law. According to the inverse square law, the intensity of the wave is inversely proportional to the square of the distance from the source.
Mathematically, the relationship can be expressed as:
I = P / (4πr²),
where I represents the intensity of the wave, P represents the power of the source, and r represents the distance from the source.
This equation shows that as the distance from the source (r) increases, the intensity of the wave (I) decreases. Specifically, the intensity decreases proportionally to the square of the distance. Therefore, doubling the distance from the source would result in the intensity being reduced to one-fourth (1/2²), and tripling the distance would reduce the intensity to one-ninth (1/3²), and so on.
It's important to note that the inverse square law assumes an idealized situation where the EM wave propagates uniformly in all directions without any obstructions or absorption by the medium through which it travels. In practice, various factors such as absorption, scattering, and interference can affect the intensity of the EM wave at different distances from the source.