PERFECTLY BLACK BODY

Explanation

Ferry designed a spherical blackbody which consists of a hollow double-walled, metallic sphere provided with a small hole on one side. The inside wall of the sphere is blackened with lamp-black while the outside is nickel-plated. The space between the two walls is evacuated to minimize heat loss by conduction and convection. Any radiation entering the sphere through the hole suffers multiple reflections where about `97%` of it is absorbed at each incidence by the coating of lampblack. The radiation is almost completerly absorved after a few internal reflections. A conical projection on the inside wall opposite the hole minimizes any chance of incident radiation escaping out.
When the sphere is placed in a bath of suitable fused salts, so as to maintain it at the desired temperature, the hole serves as a source of black-body radiation. The intensity and the nature of the radiation depend only on the temperature of the walls.
For diagram Refer to HSC paper, February `2019`, Answer `22`.