# Photon as a wave packet

1. Mar 26, 2013

### sudu.ghonge

When electromagnetic waves of different frequencies interact, they give rise to secondary wave structures called envelopes in which individual waveforms form at the rear and die out at the front. These envelopes are called groups and they travel with a velocity called group velocity and the individuals, phases and they travel with a phase velocity. These groups or envelopes are called photons right?
If so, can a photon of monochromatic EM wave exist? I recently studied that the wave function of a particle is the integral over a range of frequencies. This doesn't make sense because while studying the photo electric effect, we assume photons to be monochromatic.

2. Mar 26, 2013

### Staff: Mentor

You cannot mix classical wave descriptions and quantum mechanics like that. Photon can have some similarity to wave packets, but they do not have to. And wave packets can consist of many photons.
Only in theory, it would have to be spread out infinitely.
Well, the frequency spread is negligible for appropriate light sources.

3. Mar 26, 2013

### sudu.ghonge

So what you're saying is that even a neglible frequency range is enough to limit the spread of the group from infinity to a few nm, right?

4. Mar 26, 2013

### Staff: Mentor

Those few nm are the spread in the wavelength, not any spatial spread. This has to be at least of the order of a wavelength.

5. Mar 27, 2013

### AkInfinity

Cant have monochromatic bc according to current theory em waves are going through everything so you cant have an isolated one.

That being said.

By passing light from a star through several pinhole filters (so that only rectilinear light goes through) and then through many prisms (to defract diferent light rays) and pick out one you are looking for, and then through pinhole filters again followed by many prisms and this process then repeated you will get a monochromatic em wave. Additonally you need to do this inside a conducting sphere so that other em waves dont cross inside.