# Electromagnetic radiation and Flaw of De-Broglie Equation

## Main Question or Discussion Point

By De-Broglie,light also exibits matter property. According to him, wavelength=planck's constant/ momentum. And again momentum is the product of mass and velocity. We again know that, mass of light i.e. photon is zero. Then from De-Broglie's equation, is the wavelength of light infinity?
If so,from wave property of light,wavelength of light is finite. Is such contradiction allowed?

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The momentum of a photon is not $mv$, it's $E/c$. This follows from Einstein's $E^2=p^2c^2+m^2c^4$ for a massless particle.

(In addition, the relation $p=\gamma mv$ should be used in relativity for the three-momentum of massive particles, where m is the invariant mass, but that's not very relevant in this case. Just so you know.)

Last edited:
jtbell
Mentor
Note also that in Maxwell's classical electrodynamics (which is fully relativistic even though Maxwell knew nothing about Einstein's relativity!), the energy and momentum densities of an electromagnetic wave are related by E = pc.

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