# Kirchhoff’s law of radiation, Newton’s law of cooling, Stephen’s law

According to Kirchhoff’s law “good emitters are good absorbers”. That is, the bodies which emit more heat at a given temperature, they also absorb heat better at low temperature and good absorbers are also good emitters. In contrast, bad absorbers are bad emitters.

If we take two cups, in which the outer surface of one is rough and black and the other is shiny and white, and fill them with tea together, then we find that the tea of ​​the cup with black and rough surface cools down quickly. . This is because the black and rough surface is a good absorber of heat, so that according to Kirchhoff, it will also be a good emitter of heat. As a result, the heat loss from this cup of tea will be high and the tea will cool down quickly.

On the other hand, the white and shiny surface is a bad absorber of heat, so it will also be a bad emitter, due to which the heat loss from this cup of tea will be reduced and the tea will remain hot.

If icy cold water is poured into these two cups and kept in the sun, then the water of the cup with rough surface will heat up quickly, because now this surface will absorb more heat. Red glass appears red because it reflects red light from the light falling on it and absorbs the light of other colors.

In the seven colors of white light, except red, the combined effect of the rest of the colors is similar to that of green, so according to Kirchhoff, the emission efficiency of red will be the highest for green. This is the reason why red glass appears green when it is heated to a high temperature. On the other hand, green glass will appear red on heating.