## The Inverse Square Law for Electromagnetic Waves

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 that is 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. 