PERFECTLY BLACK BODY

Explanation

Perfectly Black body : A body which absorbs all the energy incident on it, is called perfectly blanc body,
Ferry's blace body

Construction :
1. It consist of double walled hollow metal shpere having small aperture.
2. Inner surface is coated with lamp black. The coefficient of absorption of lamp black is 98.
3. It's outer surface is nicled polished.
4. The space between sphere is evacuated.
5. There is a concial projection diametrically opposite to aperture.
Working :
(i) When radiant heat enters the sphere, it suffers multiple internal relfections.
(ii) Whenever the radiant heat falls on lamp black, about 98% of the incident radiation is abosrbed.
(iii) Hence, aftar a large number of reflections, almost all the radiant heat is absorbed by the sphere.
Thus, the aperture act as a perfectly balck body