# Planck's Radiation Law and Stefan's Law

1. Nov 24, 2009

### manofphysics

I have got 2 questions:
1)In the derivation of Planck's Radiation Law,we assume an enclosure of perfectly reflecting walls which contains diffuse radiations.These are EM waves which reflect from the walls.
Now, in my book(or even http://thermalhub.org/topics/DerivationofPlancksLaw"), it is further said that standing waves are formed which limit the wavelength to
$$\lambda=2l/n_{i}$$.
Now why are ONLY standing waves formed ?Any type of wave can be formed after reflection from the walls.Why are taking the assumption that displacement at the end of walls is zero?

2)In the derivation of Stefan's Law as given by Boltzmann, why can we apply all the thermodynamic relation and thermodyanamic laws ? Does radiation behave exactly like a gas?
I know that pressure of diffuse radiation is similar to that exerted by a ideal gas, but I STILL can't understand how and why thermodyanmics is used in radiations?

Last edited by a moderator: Apr 24, 2017
2. Nov 30, 2009

### manofphysics

I am disappointed . No reply after over 200 views.

I found out the answer myself.For all the people who didn't know,

1)This is due to Maxwell's boundary conditions at an interface.
$$E_{1}^{||}-E_{2}^{||}=0$$
where $$E_{1}, E_{2}$$ represent fields in air and conductor respectively.

2)As radiation occupies a finite space and exerts finite pressure, and hence can do work,
So we can apply thermodynamics in this case.