Waves in Fluids — Phase vs Group A packet of water waves travels as a lump (the green dot), but the individual crests (orange) move at their own speed — born at the back of the lump, sliding forward, dying at the front. Add surface tension and the crests reverse: now they slip backward instead. The journey: deep-water waves obey ω = √(g k + σk³/ρ), so speed depends on wavelength (dispersion). A crest travels at ω/k; the packet at dω/dk. For gravity waves that ratio is 2:1; capillary ripples flip it.