Question

If light does not change speed, how does luminous intensity decrease?

Answer 1

As a beam of light travels, it spreads out so that for each doubling of the distance, the intensity of the light (defined as the number of photons passing through a unit area per second) is decreased by a factor of four. This is a basic consequence of the way that light beams propagate through 3-dimensional space and has nothing to do with the speed of the beam- which does not vary with distance.

So at a point B that is distant from a light source at A, not all of the photons leaving A will hit B. The result is that fewer and fewer photons will arrive at B as the distance between A and B gets larger and larger, causing the intensity of light measured at B to decrease with increasing distance.


hope helps you