In Ferry's black body

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Space between the two walls is evacuated to prevent heat losses due to conduction & convection

A conical projection is provided on the inner wall opposite to small hole

All the above

Inner surface of inner wall is coated with lamp black and outer surface of outer wall is silver polished

answer is D.

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Detailed Solution

Perfectly black body means a body that absorbs all of radiation that falls on it.

Black surfaces (coated with lamp black) serve as good absorbers but even they are not 100% absorbers. So first what is done is a small hole is made in a sphere so as to trap all of light incident. Then a conical projection is provided on the inner wall opposite to small hole so that light reflected by it does not return back through the hole.

Space between the two walls is evacuated to prevent heat losses due to conduction & convection