Concepts of Physics

IIT JEE Physics (1978-2018: 41 Years) Topic-wise Complete Solutions

Black bodies radiate more


All black bodies emit radiation. The energy radiated by a body depends on its temperature, surface area and nature of surface.

Black painted bodies radiate more than that radiated by reflective bodies if the temperature and area are the same. This experiment makes a qualitative comparison of the heat radiated by a black and a shining body.


An electric iron, two thermometers, stand. The electric iron given is painted black at half the surface (left half if you look the surface from front) and is left in its original shining state for the other half. Place it in its common position.


  1. Put the two thermometers in their stands and place them at a distance of about 1 cm from the surface of the iron. One thermometer (the mercury part) should be in front of the black surface and the other in front of the other surface at the same distance.
  2. Now switch on the iron. Wait for few minutes and look at the two thermometers.
  3. Question: What are the temperatures of the thermometers?
  4. Question: Which thermometer has larger reading?
  5. Question: What conclusion do you draw about the emission of radiation from black and shining surfaces?


As this is the same iron, the temperature of the iron surface is almost the same at any given time. Remember, heating of the iron takes place due to the heating element inside which carries the current and not due to any radiation absorbed by the surface.