# Wavelenght of a photon

1. May 2, 2011

### ValenceE

Hello all,

The above quote is the first part of a reponse to a 2005 OP's question about frequency of a photon, and one of the few hints to an answer to one of my own enquiries about the wavelenght of a photon...

I understand that EM waves come in a vast number of frequencies, however I was wondering about the discreteness of the photon as an EM wave packet and namely the number of wavelengths it contains.

Are all photons alike in their composition, do they all have the same amount of wavelengths or is this dependant on some QM property ? if different, what determines the compositions of different wave packets ?

regards,

VE

2. May 3, 2011

### Bill_K

VE, Unfortunately Bob_for_short had it wrong.

Classically an electromagnetic wave comes in the form of a wave packet with a spread of frequencies ω ± δω. The frequency spread and the length of the packet are inversely related. One with a sharply defined frequency will have a very long wave packet, and one with a larger spread in frequency will have a short packet. Roughly the number of oscillations in the packet is given by the ratio ω/δω.

For a photon there is no difference. A photon will have some spread in energy, E ± δE. A photon with a sharply defined energy will have a long wave packet and one with a larger spread in energy will have a short one. Roughly the number of oscillations in the packet is given by E/δE.

Whether a classical description or a quantum description of the wave is appropriate depends only on the amplitude. Otherwise they are similar. The question you ask, "how many wavelengths does a photon contain" does not have a unique answer, because the answer depends on the energy spread.