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Thermodynamics

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Inverse Square Law

Consider a point source of light at S. Draw two spheres of radii R₁ and R₂. Let the source give out Q units of energy per second.

Consider a surface AB of area = S₁ and the surface CD of area = S₂

Amount of energy flowing across AB in one second,

Also amount of energy flowing across CD in one second.

Also energy flowing out of the two spheres per unit area per second is given by

The inverse square law states that the amount of light energy falling on a given surface from a point source is inversely proportional to the square of the distance between the surface and the source.

The law holds true when the source is a point. It is also true when the size of the source is very small as compared to the distance of the surface from the source.