Suppose that an area of
which forms part of the surface of a sphere of radius
receives wave energy from a point source placed at the centre. If
is the energy passing through this area of
in Watts then we say thatis the wave intensity in
in the diagram below since the power of the source,
is distributed over the surface of the sphere.
If the distance is now increased to
it is clear from the diagram above that the same power is spread over an area of
Consequently the wave intensity is reduced
Similarly when the distance is increased to
the wave intensity is
In general the wave intensity at distance
from the source is
then 
The inverse square law of the intensity of electromagnetic radiation is directly related to the fact that photons have infinite range. They do not decay as unstable atoms decay. Newton's Law of Gravity and Coulomb's Law for the force between charged particles are also inverse square law, reflecting both the infinite range of the forces and the stability of the particles that carry the force.