# Speed of Light help

1. Apr 10, 2007

### naab

Hey

If you shoot a beam of light through a piece of glass, the speed of the light changes..

When the beam exits the glass again it speeds up to the speed light has in air, right?

So my question is, where does the light get the energy to speed up again from?

2. Apr 10, 2007

### HallsofIvy

It doesn't need energy to speed up because it doesn't gain kinetic energy in speeding up (light has 0 mass and so 0 kinetic energy). The energy in a light ray depends on its frequency, not speed.

3. Apr 10, 2007

### naab

ahh okay.. thanks

4. Apr 10, 2007

### daniel_i_l

You can also look here:
https://www.physicsforums.com/showpost.php?p=899393&postcount=4 [Broken]

Last edited by a moderator: May 2, 2017
5. Apr 10, 2007

### Reshma

Also note that the speed of light referred to is the observed or measured speed in some medium and not the true speed of light (as observed in vacuum). Individual photons always travel at the same speed. The change is only apparent.

Once the light has emerged from the medium it changes back to its original speed and this is without gaining any energy. This can mean only one thing—that the light's speed itself was never altered in the first place.