Transcript: How does light travel through space? If you had a flashlight it would focus light in the forward direction. A laser beam sends light almost perfectly from one point to another. However, most astronomical sources of light or other forms of electromagnetic radiation are spherical, and they emit equally in all directions. In this situation the intensity of radiation is a function of the distance from the source, diminishing as the inverse square of the distance. This is called the inverse square law of radiation. Thus, if you moved two times further away from a light source like a star, the intensity of radiation goes down by a factor of four. Move three times further away; the intensity of radiation goes down by a factor of nine. This is a fundamental property of the way light and other forms of electromagnetic radiation diminish in intensity as you move away from a source of radiation, and it’s true for either the wave or particle descriptions. Into the wave description, the intensity of the radiation at any point goes down as the square of the distance. Under the particle description, the number of photons passing through a given area per second goes down as the square of the distance.