The relation between emissive power and absorptive power of a substance is expressed by Kirchhoff’s law. It states that the ratio between the emissive power and the absorptive power of any substance at a fixed temperature is always a constant and it is equal to the emissive power of an ideal black body at that temperature.
Application of Kirchhoff’s law:
1. A white china bowl is partially smeared with a lamp black. Now, the bowl is heated to 1000°C. If we take this bowl to a dark room then the black-coloured parts appear brighter than the rest. The black part is a better absorber than the white part. So, from Kirchhoff’s law, it is a good emitter as well. Thus, it appears brighter as it emits more heat.
2. In a solar spectrum we see many black lines. These are called Fraunhofer lines. The origin of these lines can be explained with the help of Kirchhoff’s law.