# How does the speed of light change when it enters glass?

• richengle
In summary, the conversation discusses the concept of light slowing down in glass and the different velocities associated with it, including phase velocity, group velocity, and the velocity of individual photons. References are provided for further understanding, and the difference between phase and group velocity is explained. The conversation also briefly mentions the Kelvin Wedge as an example of the difference between the two velocities.f

#### richengle

its has been said that light slows down in glass, but some talk about phase velocity and group velocity... then there is the velocity of the photons, which some say is always c.

What are those speeds [Vgroup, Vphase, Vphoton, Vlight] in glass, and what do they mean?

MinutePhysics explains it well:

Summary:: light is said to slow down when it enters glass, but some say no. some say the photons travel at c, but the light wave propagates at 1/sqrt(ue). what are the velocities?

its has been said that light slows down in glass, but some talk about phase velocity and group velocity... then there is the velocity of the photons, which some say is always c.

What are those speeds [Vgroup, Vphase, Vphoton, Vlight] in glass, and what do they mean?
Just to place your question in context so that it can be answered more to the point, can you provide references for all these "somes" that "say" all of that?

vanhees71 and jedishrfu
Sixty Symbols has a nice pair of explanations in this YouTube video:

MinutePhysics explains it well:

Nope. This would be like diffuse transmission. It's just not an "atom-by-atom" process. It's the effect of all atoms collectively.

Nope. This would be like diffuse transmission. It's just not an "atom-by-atom" process. It's the effect of all atoms collectively.
yeah, but then if, as below stated 60 sec video, this is not acurate, then what are the real velocities?

Sixty Symbols has a nice pair of explanations in this YouTube video:

here is the second video of the series;;;

What are those speeds [Vgroup, Vphase, Vphoton, Vlight] in glass, and what do they mean?
I have no idea what "they" mean.

Basically there is phase velocity and group velocity, although some may give them other names. The easy way to sort these out is that phase velocity is the propagation velocity of individual photons, and group velocity is the propagation velocity of a bunch of photons in some arrangement, like a pulse of light. The actual calculations are difficult.

https://en.wikipedia.org/wiki/Group_velocity

The easy way to sort these out is that phase velocity is the propagation velocity of individual photons, and group velocity is the propagation velocity of a bunch of photons in some arrangement, like a pulse of light. The actual calculations are difficult.
One should leave photons completely out of any discussion of phase or group velocity. The terms are perfectly applicable to classical waves and should be understood in that context.

The Wiki article that you reference does not use the term photon at all.

The phase velocity is the velocity of a particular point on a wave. For instance, a peak or a trough. The term "phase" is used because a peak or a trough is an example of a "phase".

The group velocity is the velocity of a group of waves moving together, hence the name. The group velocity and the phase velocity can differ because the group may evolve over time with trailing waves increasing in amplitude while leading waves fade away. In such a case, the phase velocity will exceed the group velocity. The group will lag behind.

If you look carefully at the bow wave of a ship or boat you can witness the difference in the two velocities first hand. You can see that the wave fronts defined by a particular phase within the bow wave are diagonal to the front defined by the leading edge of the disturbance that is the bow wave.

Sadly, a quick trip to Google did not bring up any good images of this.

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DaveE, vanhees71 and sophiecentaur
Do you mean the Kelvin Wedge?

jbriggs444 and vanhees71
Sadly, a quick trip to Google did not bring up any good images of this.
You mean like this?

"The red square moves with the phase velocity, and the green circles propagate with the group velocity."

From: https://en.wikipedia.org/wiki/Group_velocity

DaveE, DrStupid, vanhees71 and 1 other person
Not what I had in mind., @A.T. But quite a bit better. Directly on point.

vanhees71