Planck's Radiation Law and Stefan's Law

  • Thread starter Thread starter manofphysics
  • Start date Start date
  • Tags Tags
    Law Radiation
AI Thread Summary
The discussion addresses two key questions about Planck's Radiation Law and Stefan's Law. It explains that only standing waves form in a perfectly reflecting enclosure due to Maxwell's boundary conditions, which dictate that the electric field must be continuous at the interface. The second question clarifies that thermodynamic principles can be applied to radiation because it occupies a finite space, exerts pressure, and can perform work, similar to an ideal gas. The original poster found answers to their questions, highlighting the importance of boundary conditions and the behavior of radiation in thermodynamics. Understanding these concepts is crucial for grasping the derivations of these fundamental laws in physics.
manofphysics
Messages
41
Reaction score
0
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:
Science news on Phys.org
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.
 
Back
Top