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The average energy density ##d\psi## is obtained by dividing the radiant energy ##dE## received by the surface ##dB## in 1 second by the cylindrical volume ##dV## occupied by the radiation in 1 second.

I don't understand

**why ##d\psi## is uniform anywhere inside the cylinder**, in particular, at points near the surface ##dS##. At a point far away from ##dS##, the point is always the same distance ##\rho## away from every part of ##dS##. But this is not true for a point near ##dS##. The point would be closer to some parts of ##dS##. Then the energy density ##d\psi## at the point should not be uniform but vary depending on how far the point is from the center of the surface ##dS##.