# Photon acceleration(or acceleration wrt photons)

1. Sep 18, 2004

### Silverious

When a photon changes mediums it's speed wrt to c changes correct?

So dv/dt = a (d as in delta). So how long does it take a photon to "pass through" one medium into another, and is this considered acceleration.

Also if it doesn't "pass through" one medium to another(meaning it isn't like shooting a bullet into water, instead it just "is") then how CAN you determine the dv if there is no dt?

And who accelerates? The photons or the medium?

2. Sep 18, 2004

Staff Emeritus
If you insist on using photons, the change is intantaneous, since the photon is massless. But really, the wave nature of light is better adapted to the subject of refraction.

3. Sep 18, 2004

### Silverious

I don't know why but I just don't really buy it.

There's got to be something else going on.

Photons also have energy though. How is this effected by it's speed? And how does that effect vary as it moves between mediums?

4. Sep 18, 2004

### ZapperZ

Staff Emeritus
Photons DO NOT slow down in a medium!

The apparent decrease of light (as in the wave nature of light) in a medium is when we measure the GROUP VELOCITY of light, which is the only way we can make a determination of what is being measured. It is this group velocity that changes upon entering and leaving a medium. In a vacuum, the phase velocity and group velocity are the same.

Zz.

5. Sep 18, 2004

### Creator

Silverious;
The energy (frequency) of a photon doesn't need to change. You can see it easier in terms of waves; the reason the speed changes in different media is not due to an energy (frequency) change, but rather due to a change in WAVELENGTH. Since c = wavelength x freq., then c can change as wavelength changes without a change in frequency.

Creator

Last edited: Sep 18, 2004
6. Sep 23, 2004

### Silverious

New Question: How about when they are reflected off a mirror? Does acceleration not occur then?

7. Sep 23, 2004

### humanino

Do you think photons are like tennis balls ? It is not the same photons in one direction and in the other.