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: [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?