The inverse square law shows that when light travels twice the distance its area grows four times as large and the brightness decreases by four times. As the light travels it has a specific brightness and size at any given point. The figure shows directional light originating from a point source that covers a larger area the further away it is from the source.

Because the same geometry applies to many other physical phenomena (sound, gravity, electrostatic interactions), the inverse square law has significance for many problems in physics.

As you move away from a point light source, the intensity of the light is proportional to 1/ r 2, the inverse square of the distance. This is what is meant by the inverse square law. Thus, at three times the original distance, the intensity of the light passing through a single square will be 1/9 of the original intensity. Going out still farther, tripling the original distance ( 3r), and the light from the original square now covers an area of 9 (= 3 2) squares. Thus, at twice the original distance, the intensity (power per square meter) of the light passing through a single square will be 1/4 of the original intensity. The light from the original square has now 'spread out' over an area of 4 (= 2 2) squares. Move away, doubling the distance from the star ( 2r). Now imagine the light that falls on a square at some arbitrary distance from the star ( r). Imagine the light from the star spreading out into empty space in all directions. The blue area, marked 'S,' represents a point source of light. No doubt you have noticed this with reading lamps, streetlights, and so on. As you move away from a light source, the light gets dimmer.

Intensity of light equation power