Applications of Kirchhoff’s Law
According to Kirchhoff’s law the ratio of emissive power to absorptive power is same for all surfaces at the same temperature and is equal to the emissive power of a perfectly black body at that temperature.
Applications:
1) Sand is rough black, so it is a good absorber and hence in deserts, days will be very hot. Now in accordance with Kirchhoff’s law, good absorber is a good emitter so nights will be cold. This is why days are hot and nights are cold in desert.
2) Sodium vapors, on heating, emit two bright yellow lines. These are called \({{D}_{1}}\), \({{D}_{2}}\) lines of sodium. When continuous white light from an arc lamp is made to pass through sodium vapors at low temperature, the continuous spectrum is intercepted by two dark lines exactly in the same places as \({{D}_{1}}\) and \({{D}_{2}}\) lines. Hence sodium vapors when cold, absorbs the same wavelength, as they emit while hot. This is in accordance with Kirchhoff’s law.
3) When a shining metal ball having some black spots on its surface is heated to a high temperature and is seen in dark, the black spots shine brightly and the shining ball becomes dull or invisible. The reason is that the black spots on heating absorb radiation and so emit these in dark while the polished shining part reflects radiations and absorb nothing and so does not emit radiations and becomes invisible in the dark.
4) When a green glass is heated in furnace and taken out, it is found to glow with red light. This is because red and green are complimentary colors. At ordinary temperatures, a green glass appears green, because it transmits green color and absorbs red color strongly. According to Kirchhoff’s law, this green glass, on heating must emit the red color, which is absorbed strongly. Similarly when a red glass is heated to a high temperature it will glow with green light.
5) A person with black skin experiences more heat and colder as compared to a person of white skin because when the outside temperature is greater, the person with black skin absorbs more heat and when the outside temperature is less the person with black skin radiates more energy.
6) Kirchhoff’ law also explains the Fraunhofer lines:
i) Sun’s inner most part emits radiation of all wavelengths at high temperature.
ii) When this radiation enters in outer part of sun, few wavelengths are absorbed by some terrestrial elements.