Brightness of Electromangetic Waves
The surface area covered by an expanding sphere of light (or a portion of it) increases as the square of the radius of the sphere, that is, as the square of the distance from the light source. Since the total amount of energy leaving the light source in all directions is the same at any distance, the amount of radiation passing through each unit of area of the expanding sphere must diminish with the square of the distance.
For example suppose at 1 meter from a light source that the apparent brightness of the radiation over 1 square meter of surface area is 1 unit. At twice the distance each square meter will receive one-fourth of a unit of illumination; at three times the distance,
INVERSE-SQUARE LAW OF LIGHT: The apparent brightness b varies inversely as the square of the distance d from the light source; that is, b x 1/d2•
