Speed of light : missing energy

**timharvey027** · Nov21-09, 11:44 AM

consider light traveling through a block of glass, when it enters the block it slows down , right and is traveling at say spped of light minus X. When it emerges from the block, it is back to traveling at the speed of light. Question: At the exact point light leaves the block it must accelerate..to get to the speed of light. In order to accelerate it must get energy from somewhere...where does it get this energy???
Considering light either as a wave, stream of photon or vibrating strings it still needs more energy than it had when traveling in glass where does it come from

**f95toli** · Nov21-09, 11:48 AM

Light never accelerates. It ALWAYS travels at the same speed.
Also, it is only the effective speed of light (for lack of a better word) that is lower in glass, the photons are still traveling at the same speed as before they entered the block; it is just that they interact with the ions/electrons in the solid but between these intereractions they are traveling at c.

**A.T.** · Nov21-09, 11:50 AM

And the energy of light doesn't depend on it's propagation speed.

**IPhO' 2008** · Nov22-09, 03:35 AM

c = $LaTeX Code: \\sqrt{\\frac{1}{\\mu\\epsilon}}$
In the medium $LaTeX Code: \\epsilon$ , $LaTeX Code: \\mu$ increases.
So,c decreases.

**Lsos** · Nov25-09, 07:40 AM

I'm no expert on the subject, and I'm not exactly sure what happens in glass....but either way light has no mass. So looking at F=ma, it becomes clear that the acceleration can easily be instantaneous.

This is probably a gross oversimplification of what happens, and it's more likely that F=ma simply doesn't apply here. Which probably also means that the simple way you're thinking about the problem also doesn't apply.

Maybe that doesn't answer the question, but at least it might help you realize that you might be thinking "in the box" :)

**MikeyW** · Nov25-09, 08:16 AM

Since the presence of a dielectric reduces the net electric field, how can the intensity of radiation (I proportional to E^2) not fall?

When a photon is in a dielectric, it does work to orient the atomic dipoles in the opposite direction, right?

**Ikoro** · Nov25-09, 10:07 PM

@ Mikey: Well the photon does not work to orient remember it is like induction but in this case actiona t a distance. The glass itself actually orients its particles because of the presence of the photons so it could be attracted ...so in actuality it does no work.