The Mysteries of Light Velocity

[androz]

Light velocity...

Hi,

There is a question that I can't figure out.

What's the difference between signal and group velocity for a light pulse ?

What I understood so far is that the signal velocity is the velocity of the first detectable photon and the groupe velocity is the velocity of the pulse peak.

Am I right ?

 

[Gonzolo]

There are definitions here :

http://www.uni-koeln.de/~abb11/workshop/announce4.html

Group velocity is what it has always been : domega/dk. Signal velocity seems to be a little more delicate to define.

 

[R0man]

Thanks for your question! The difference between signal velocity and group velocity for a light pulse is an important concept in understanding the behavior of light. Signal velocity refers to the velocity of a single photon, while group velocity refers to the velocity of a pulse of light, which is made up of multiple photons.

You are correct in your understanding that signal velocity is the velocity of the first detectable photon, while group velocity is the velocity of the pulse peak. However, it is important to note that the speed of light is constant in a vacuum and is always measured to be approximately 299,792,458 meters per second. This means that the signal velocity and group velocity for a light pulse will also be the same in a vacuum.

The difference between signal velocity and group velocity becomes relevant when light travels through a medium, such as air or water. In these mediums, the speed of light is slower than in a vacuum, due to interactions with the atoms and molecules in the medium. This difference in speed can cause the signal velocity and group velocity to be different. The signal velocity will be slower than the speed of light in a vacuum, while the group velocity may be faster.

In summary, the difference between signal velocity and group velocity for a light pulse is that signal velocity refers to the velocity of a single photon, while group velocity refers to the velocity of a pulse of light made up of multiple photons. This difference becomes more apparent when light travels through a medium with a slower speed than in a vacuum. I hope this helps clarify the concept for you.