question_answer

Which of the following is close to an ideal black body:                                           [AIPMT 2002]

A)  black lamp

B)                               cavity maintained at constant temperature

C)       platinum black  

D)       a lamp of charcoal heated to high temperature

Correct Answer: B

Solution :

Materials like black velvet or lamp black come close to ideal black bodies, but the best practical realization of an ideal black body is a small hole leading into a cavity maintained at constant temperature as this absorbs 98% of the radiation incident on them. Cavity approximating an ideal black body is shown in the figure. Radiation entering the cavity has little chance of leaving before it is completely absorbed.