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Relating Intensity to Radial Distance
It may be the case that this derivation is not required by your particular syllabus. However, understanding how equations are derived from first principles will give you a deeper understanding of physics. This indirectly will probably lead to a better result.
The law states:
The intensity is inversely proportional to the square of the displacement.
Consider electromagnetic radiation emanating in all directions from a source. At a distance x from the source the energy is spread over a sphere of radius x and area 4Πx2 .
If E is the energy radiated from the source in all directions per second, then the intensity I is the total energy/second divided by the area it is spread over.
The intensity I has units of energy per unit time per unit area( unit J s-1m-2 ).
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