Which term is used for the speed at which the energy in a wave group travels?

• A. Destructive Interference
• B. Group Velocity
• C. Dispersion
• D. Phase Velocity

Answer: (B)

The energy in a wave group travels at the group velocity. This is the speed of the envelope of a wave packet—the overall shape that carries the energy and information. It differs from the speed of the individual wave crests, which is the phase velocity (ω/k). In dispersive media, these two speeds can differ, so energy moves with the group velocity while the crests move at the phase velocity. In non-dispersive media, group and phase velocities are the same, so energy and crests travel together. Destructive interference isn’t about speed, it’s about how wave amplitudes combine. Dispersion describes how different frequencies move at different speeds and cause the packet to spread, which is why the group velocity can differ from the phase velocity. In deep-water gravity waves, for example, the group velocity is half the phase velocity, illustrating the distinction.