The Inverse Square Law:
The
intensity (or
illuminance or
irradiance) of
light or other linear waves radiating from a
point source (energy per unit of area perpendicular to the source) is inversely proportional to the square of the distance from the source so an object (of the same size) twice as far away, receives only ¼ the
energy (in the same time period).
I stole the definition from wiki. In photo it's an important rule of thumb to keep in mind because it describes the falloff of the light as you move the source from the object it's illuminating.