Suppose that an area ofwhich forms part of the surface of a sphere of radiusreceives wave energy from a point source placed at the centre. Ifis the energy passing through this area ofin 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 toit 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 tothe wave intensity is In general the wave intensity at distancefrom the source isthen

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.