When any kind of radiation (radioactive particles from a source, or electromagnetic waves) is incident on a surface, its irradiance, , is defined as the power per unit area. This relationship is represented by the following equation:
As the distance from a point source of radiation increases, the irradiance decreases. The relationship between irradiance, , and distance, , can be shown to follow an inverse square law.
The product of irradiance and the square of the distance from the source is a constant, .
As this product is constant, it follows that for two points at distances and from a point source of radiation: