Phase Velocity & Group Velocity
Sinusoidal waves They are Progressive Wave This is standing wave
T t
x
New position of at
Phase velocity =The speed with which the constant phase moves
Group velocity
Phase velocity Group velocity
vp < vg t = 0
vp < vg t = 1
vp < vg t = 2
vp < vg t = 3
vp < vg t = 4
vp < vg t = 5
vp < vg t = 6
vp < vg t = 7
vp < vg t = 8
vp < vg t = 9
vp < vg t = 10
vp = vg t = 0
vp = vg t = 1
vp = vg t = 2
vp = vg t = 3
vp = vg t = 4
vp = vg t = 5
vp = vg t = 6
vp = vg t = 7
vp = vg t = 8
vp = vg t = 9
vp = vg t = 10
vp > vg t = 0
vp > vg t = 1
vp > vg t = 2
vp > vg t = 3
vp > vg t = 4
vp > vg t = 5
vp > vg t = 6
vp > vg t = 7
vp > vg t = 8
vp > vg t = 9
vp > vg t = 10
t2 t0 t1 Non dispersive: All colours moving with same speed Dispersive: Red moving faster than blue
Phase and Group velocity vg P’
vp = Constant Non dispersive waves Signal is propagated without distortion More generally vp is a function of (or k)
Electromagnetic waves in Ionosphere, wave guides
Water waves = density = surface tension h = depth of water
term and T term are comparable
Long waves in deep water
Short ripples under surface tension
Long waves in Shallow water [ ]kh Vg ≈ Vp Tsunami