# Archived Planck's law and doppler effect

1. May 4, 2013

### Unicorn.

Hello, We proved something during the course but I totally forgot how to do it..
1. The problem statement, all variables and given/known data
Prove that Planck's Law E=h*u is deduced from the equation of Doppler effect u'=u*√((1-β)/(1+β))
2. Relevant equations
Lorentz transformation

3. The attempt at a solution
If we take a beam of light of frequency u and an observer with speed Bc
I don't know if it's correct..
u'=u√((1-β)/(1+β))
E'=y(E-B*Px*c)
And px*c=E so
E'=y(E-BE)
=yE(1-B)
=E√((1-β))/(√((1-β)/(1+β)))
=E√((1-β))/√(1+β)
E'=E*u'/u
E'*u=E*u'
E'/u'=E/u
E/u=cste=h ?

Last edited: May 5, 2013
2. Jul 3, 2016

### James R

I don't think that the energy of a photon can be derived solely from a consideration of the Doppler effect (relativistic or not). More information is needed.