understanding for group velocity and phase velocity?? Hi, everyone. I learned physics for 3 years, but up to now I still cannot understand clearly the difference between the group & phase velocity. What is exatly "phase"? and what is exactly "group". From book I read the Vph corresponds to the transmisssion of phase and the Vg corresponds to the transmission of node or based on the velocity of signal modulation by some approximation. But still feel abstract about these words. Also, if we use a light source and a detector locate at two position with distance Zo, and put a material between them. and in some cases Vph could <Co. However the result by using this material is just the delayed phase of the signal. What does this mean? By what method could we observe this less velocity? Does this mean that the time duration from the source to the signal would smaller than Zo/Co? Many thanks!