Which of the following is best close to an ideal black body : -
A
Black lamp
B
Cavity maintained at constant temperature
C
Platinum black
D
A lump of charcoal heated to high temp.
The answer is B.
Solution
An ideal black body absorbs all radiation falling on it, of every wavelength, and is also the best emitter. A cavity (enclosure) with a small hole, kept at a constant temperature, is the closest practical realisation (Ferry's black body). Radiation entering the hole is reflected many times inside and is almost completely absorbed. The radiation coming out of the hole is very nearly ideal black body radiation characteristic of the temperature of the cavity. Lamp black and platinum black absorb about 96–99 percent, so they are good but not ideal. So the best approximation to an ideal black body is a cavity maintained at constant temperature.
ConceptCavity radiator as ideal black body
Thermal Properties of Matter in past papers
49 questions from this chapter have appeared across 25 exam years.
