# Speed light in a medium

1. Aug 3, 2009

### lolgarithms

I have heard that the speed of light remains c even if it is not in a vacuum. the light bounces around in the substance a bit, so light appears to have slowed down. Is this true?

2. Aug 3, 2009

### Bob S

The speed of light c in a medium with index of refraction n is c/n. For water, n = 1.33, and for glass, it is about 1.52 to 1.54. The index of refraction depends slightly on the wavelength of light (called dispersion), so we see light spectra from glass prisms, or in rainbows.

3. Aug 3, 2009

### DrGreg

When photons of light pass through a material, each photon still travels at c, but when it hits an atom, it gets absorbed. Then there's a short delay before another photon is emitted, at c, continuing in the same direction. So the light gets delayed through the medium and the average speed is less than c.

That's the over-simplified version, easy to understand, but actually not true in quantum-mechanical terms. For the full story, see the https://www.physicsforums.com/showpost.php?p=899393&postcount=4 [Broken]

Last edited by a moderator: May 4, 2017
4. Aug 3, 2009

### Bob S

This is true on a microscopic level, but not what we tell high school or college freshmen physics students.

5. Sep 24, 2009

### DeepQ

That's a good reference, thanks. But, it would also be nice to see that full story expanded beyond solids to include liquids and gases. Certainly, density plays a roll in the available phononic modes, but it would be interesting to understand, for example, how water obtains its index.

Last edited by a moderator: May 4, 2017
6. Sep 24, 2009

### raphael3d

i wonder how far one can slow down c in various solid state materials and i little thought experiment: what if n -> infinity ? would that imply that the transparent material is so dense, that light would take an infinit long time to pass?

7. Sep 24, 2009

### ZapperZ

Staff Emeritus
You must have missed the report out of Lene Hau's group at Harvard from several years ago. How about slowing down light until it "stops"?

While this isn't a "solid state material", it tells you what happened and what we mean by "stopping light", as opposed to light being absorbed or stopped by an opaque object.

Zz.

Last edited by a moderator: May 4, 2017
8. Sep 24, 2009

### raphael3d

yes missed it.
many thanks for the link, very interesting indeed.

but it hasnt anything to do with slowing down photons at all, rather using tricks which seem to have the same effect, like quantum beaming, which isnt really beaming, more transforming states. same here.

the bose-einstein-condensate is a bit closer to what i want, but the process is described above...again kind of a trick. it seems to me that photons aways travel with 3x10^8 m/s but with different times of arrival in different mediums. by deforming space too, light follows a longer path, but with the same velocity it takes longer to arrive at some point. the bottom line is in every example longer times in traveling but with constant velocity c
please correct me if i have written ******** =)

kind regards

9. Sep 24, 2009

### ZapperZ

Staff Emeritus
There's no such thing as "slowing photons" since photons always move at c (read our FAQ in the General Physics forum). What you are measuring when we talk about the "speed of light" is the speed of the GROUP VELOCITY. That is why I specifically stated that that link clearly explains what we mean by stopping light.

Zz.

Last edited: Sep 25, 2009
10. Sep 25, 2009

### raphael3d

think we agree, just wanted to clear things up.

thanks again.