# Does light travel at different speeds?

1. Jun 7, 2006

### Burnsys

Controlling the Speed of Light -- Up and Down
http://www.primidi.com/2005/08/20.html

Hi!, i read everywere that someone has "Slowed down" light, speeded it past the speed of light etc.

Correct me if i am wrong, but, isn't the speed of light a constant?????
When the light is "slowed down" it actualy never slows down, it is just absorbed by the atoms and then emited, but light never slow down.

I am tied of this kind of articles, each time they are out, you have a lot of people totaly confused about light and special relativity

Last edited: Jun 7, 2006
2. Jun 7, 2006

### J77

The exact speed of light depends on the type of medium it's propagating in.

eg. c=3e8 m/s in a vacuum.

Though I could be wrong here - this is just what I understand by it...

3. Jun 7, 2006

### Burnsys

right but can light travel in other medium that is not a vaccum???

I mean what medium is a piece of glass?? a collection of atoms, can light travel inside an atom??? can light propagate on mater?

Edit: This was discused here before: https://www.physicsforums.com/showthread.php?t=122920

Last edited: Jun 7, 2006
4. Jun 7, 2006

5. Jun 7, 2006

### Staff: Mentor

The simple answer is yes photons only ever travel at C. The mechanism of refraction is complicated, though.

6. Jun 8, 2006

### reidy

i heard c=c/n
where n is the refractive index of the material it is propagates through

air= 1.0003
water=1.4

dont know if this is correct

7. Jun 8, 2006

### dimensionless

Generally speaking the speed that the light propagates through matter is slower.

I think the idea that they might light travel faster is misleading. Wasn't it only the phase velocity that they were able to accelerate? The phenomenon was accomplished with x-rays some 60 years ago.

8. Jun 9, 2006

### Burnsys

I know, but, what is matter??? an electron? an atom? a proton?? does light realy propagates through matter?? can light travel through a proton or an electron? or it's just absorbed and emited? in the last case it's not traveling through mater the photon just ceased to exist and later it was created again. ( i think )