Question
The total radiant energy per unit area, normal to the direction of incidence, received at a distance R from the centre of a star of radius r, whose outer surface radiates as a black body at a temperature T K is given by
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σr2T4/R2
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σr2T4/4πR2
-
σr4T4/r4
Solution
A.
σr2T4/R2
If r is the radius of the star and T its temperature, then the energy emitted by the star per second through radiation in accordance with Steafan's law will be given by
AσT4 = 4πr2σ T4
In reaching a distance R this energy will spread over a sphere of radius R; so the intensity of radiation will be given by 
