Question

In a dark room with ambient temperature To​, a black body is kept at a temperature T. Keeping the temperature of the black body constant (at T), sun rays are allowed to fall on the black body through a hole in the roof of the dark room. Assuming that there is no change in the ambient temperature of the room, which of the following statement(s) is/are correct?
(THIS QUESTION HAS MULTIPLE CORRECT OPTIONS)
A) The quantity of radiation absorbed by the black body in unit time will increase
B) Since emissivity = absorptivity, hence the quantity of radiation emitted by black body in unit time will increase
C) Black body radiates more energy in unit time in the visible spectrum.
D) The reflected energy in unit time by the black body remains the same.

Answer 1

Hint:The atmospheric temperature is the average air temperature surrounding a person whether it is inside or outside. In relation to weather, the atmospheric temperature is the same as the current air temperature at any one location.

Complete step by solution:
The bodies which radiate energy in the form of photons can be determined using Kirchhoff’s law of radiation. When these photons reach another surface, they may either be absorbed, reflected or transmitted.
Since the radiation is continuously falling on the black body, the quantity radiation absorbed per second will increase.
The reason given in question is self-explanatory. With an increase in temperature, the entire Plank's curve shifts upwards and hence radiation in any spectrum will increase. Reflected energy per unit time will be zero since black body has zero reflectivity and hence it will remain constant.
 Therefore, the quantity of radiation absorbed by the black body in unit time will increase, since emissivity = absorptivity, hence the quantity of radiation emitted by black body in unit time will increase, Black body radiates more energy in unit time in the visible spectrum, the reflected energy in unit time by the black body remains same.

Hence, the option (A), (B), (C) and (D) are the correct answer.

Note:The primary law which governs the radiation is the Planck Radiation Law, which gives that the intensity of radiation emitted by a black coloured body as a function of wavelength for a fixed temperature. The Planck law gives a distribution, which peaks at some wavelength.