Q.Draw a neat labelled diagram of Ferry's perfectly black body. Compare the rms speed of hydrogen molecules at 227°C with rms speed of oxygen molecule at 127°C. Given that molecular masses of hydrogen and oxygen are 2 and 32 respectively.

Ferry's black body traps radiation via a small hole in a blackened cavity; rms speed depends on $\sqrt{T/M}$.

Ferry's perfectly black body: a hollow double-walled sphere with the inner surface coated with lamp-black, with a small opening (hole) on one side through which radiation enters. Directly opposite the hole, a conical projection is built into the inner wall. Radiation entering the hole strikes the cone and undergoes multiple internal reflections/absorptions on the blackened inner surface, so that essentially none of it escapes back out through the hole — the hole itself behaves as a near-perfect absorber (and, when the sphere is heated, as a near-perfect emitter), realising an ideal black body.

$\text{[Diagram: double-walled blackened hollow sphere with a small aperture, and an inward conical projection opposite the aperture]}$

rms speed comparison: $v_{rms} = \sqrt{\dfrac{3RT}{M}}$, so …