The inverse square law states that the intensity of a light source is inversely proportional to the square of its distance from the source. In other words, for every unit increase in distance, the intensity decreases by a factor of two. This law applies to any type of light source including sunlight, incandescent bulbs and LEDs. It is also applicable to sound waves, gravity and electric fields. The inverse square law is used to calculate and analyze light output, power levels and other factors related to illumination and sound propagation.