# A Kirchhoff law for absorption and emission of a gas

1. Aug 26, 2016

### Carlos de Meo

Hi Guys
I´m studying the absorption and emission phenomena of a gas and, according to Kirchhoff "extended" law, for a body at thermodynamic equilibrium, the following statement is true.
ελ(T,θ,φ) = Aλ(T, θ,φ).
So, if i understood correctly the meaning of this equation, The amount of absorbed energy must be emitted with the same probability at all direction at the same wavelength, is it correct?
If so, the only difference between reflection and emission at equilibrium (in therms of energy) is that i´m able to predict the direction of the reflected photon (assuming specular reflection) and for the absorption/emission phenomena, i can only say that it happens with the same probability through all direction?
Thank you very much for your time and patience

2. Aug 26, 2016

### Staff: Mentor

Could you please give more details of the physical system you are considering?

3. Aug 26, 2016

### Carlos de Meo

Oh sure, sorry about that.
If we think an enclosure containing a gas inside. This system is already at thermodynamic equilibrium. The walls are reflecting, absorbing and emitting photons,the gas is absorbing and emitting (lets not talk about scattering at the moment). So if i apply the Kirchhoff equation for this system, it says that when a photon is absorbed, it should be emitted through all direction with the same probability at the same wavelength that it was absorbed?

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