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Consider a spherical shell of radius R at temperature T. The black body radiation inside it can be considered as an ideal gas of photons with internal energy per unit volume u = V U ∝ T 4 and pressure p = 3 1 ⎟ ⎠ ⎞ ⎜ ⎝ ⎛ V U . If the shell now undergoes an adiabatic expansion the relation between T and R is -

1. T ∝ e –R
2. T ∝ e –3R
3. T ∝ R 1
4. T ∝ 3 R 1

Correct answer: T ∝ R 1

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