Nov. 4, 2019

Kirchhoff’s Law:

Kirchhoff’s Law

According to this 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 balck body at that temperature. 

Hence e1/a1 = e2/a2 = ….. (E/A) Perfectly balck bdoy

A = 1, i.e., e/a = E

    ( eλ/aλ)= (Eλ)black

Now since (eλ)black  is constant at a given temperature, according to this law if a surface is a good absorber of a particular wavelength it is also a good emitter of that wavelength.

This in turn implies that a good absorber is a good emitter (or radiator)

Applications of Kirchhoff’s Law

(1) sand is rough and black, so it is a good absorber and hence in deserts, days (when radiation from the sun is incident on snd) will be very bot.

(2) Sodium vapous, on heating, emit two bright yellow lines. These are called D1, D2 lines of sodium. When continuos white light form an arc lamp is made to pass through sodium vapours at low temperature, the continuous spectrum is intercepted by two dark lines exactly in the same places as D1, and Dlines. Hence sodium vapours when cold, absorbes the same wavelength, as they emit while hot. This is in accordance with Kirchhoff’s law.