Planck considered the black body radiations (in the hohlraum) to consist of linear oscillators of molecular dimensions and that the energy of a linear oscillator can assume only the discrete values

                                         0,hv,2hv,3hv....nhv                                                
If N_0, N₁,N_2....are the number of oscillators per unit volume of the hologram possessing energies0,hv,2hv....respectively, then the total number of oscillators N per unit volume will be
                                          N = N₀+N₁₊N₂+.....

But the number of oscillators, N_r having energy E_r is given by (Maxwell’s formula)
                                          N_r = N₀

Putting these values in eqn., we get
                      N = N0 +N0+N0+......N0
                          = N0(1+++.....)
                         
The total energy of N oscillators will be   
                                                  
Hence the average energy per oscillator is
                          
                  (on dividing numerator and denominator bye-hv/Kt)
Thus we see that the average energy of the oscillator is not Kt (as given by classical theory)but equal to hv/(ehv/kt-1) according to Planck’s quantum theory.
Further, it can be deduced that he number of oscillators per unit volume having frequent in the range of v and v+dv is equal to
                       

Hence the average energy per unit value (i.e., energy density) inside the enclosure is obtained by multiplying with i.e. it is given by
                             

Putting      and    ,the average energy per unit volume in the enclosure of the wave lights between    and +d will be
                             
Or the energy radiated by the black body corresponding to wavelength     is

                              
Which is Planck’s radiation law or Pluck’s distribution law?
  The above equation is also quite often written in the form
                               
where  and  c₂=hc/k are universal constants.

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