I'm currently working with lasers and in relation to that, fiber optics. In the book I'm reading there's a section regarding modes and their "group velocity". The text claims that there is the following relation:(adsbygoogle = window.adsbygoogle || []).push({});

[tex] v_g=v_p * sin(A) [/tex]

where [tex] v_g [/tex] is the group velocity and

where [tex] v_p [/tex] is the phase velocity and A is the incident angel as showed on the picture, although denoted as theta there.

From the picture you can see that the relation is reached by using trigonometry, and [tex] v_g [/tex] is in this case simply the velocity in the direction along the fiber. My question is, if this is the general idea of group velocity? I thought group velocity was the velocity of the "amplitude wave" in a wave packet? Am I wrong, or are there just different meanings of group velocity?

**Physics Forums | Science Articles, Homework Help, Discussion**

Join Physics Forums Today!

The friendliest, high quality science and math community on the planet! Everyone who loves science is here!

The friendliest, high quality science and math community on the planet! Everyone who loves science is here!

# Need some help regarding phase and group velocity of a wave

**Physics Forums | Science Articles, Homework Help, Discussion**