1. Phase Velocity and Group Velocity
Depinder Singh
Assistant Professor of Physics
Govt. College Ropar

2. Particle ______________________________
Our traditional understanding of a particle… “Localized” - definite position, momentum, confined in space

3. Wave ____________________________
Our traditional understanding of a wave…. “de-localized” – spread out in space and time

4. Waves Normal Waves Matter Waves are a disturbance in space
carry energy from one place to another
often (but not always) will (approximately) obey the classical wave equation
Matter Waves
disturbance is the wave function Y(x, y, z, t )
probability amplitude Y
probability density p(x, y, z, t ) =|Y|2

5. General Wave properties

6. What could represent both wave and particle?
How do we associate a wave nature to a particle? ___________________________________
What could represent both wave and particle?
Find a description of a particle which is consistent with our notion of both particles and waves……
Fits the “wave” description
“Localized” in space

7. ____________________________________
A “Wave Packet”
How do you construct a wave packet?

8. What happens when you add up waves? ________________________________
The Superposition principle

9. Adding up waves of different frequencies

10. Constructing a wave packet by adding up several waves ………… ___________________________________
If several waves of different wavelengths (frequencies) and phases are superposed together, one would get a resultant which is a localized wave packet

11. A wave packet describes a particle ____________________________
A wave packet is a group of waves with slightly different wavelengths interfering with one another in a way that the amplitude of the group (envelope) is non-zero only in the neighbourhood of the particle
A wave packet is localized – a good representation for a particle!

12. Wave packet, phase velocity and group velocity ____________________________
The spread of wave packet in wavelength depends on the required degree of localization in space – the central wavelength is given by
What is the velocity of the wave packet?

13. Wave packet, phase velocity and group velocity
The velocities of the individual waves which superpose to produce the wave packet representing the particle are different - the wave packet as a whole has a different velocity from the waves that comprise it
Phase velocity: The rate at which the phase of the wave propagates in space
Group velocity: The rate at which the envelope of the wave packet propagates

14. Phase velocity
Phase velocity is the rate at which the phase of the wave propagates in space.
This is the velocity at which the phase of any one frequency component of the wave will propagate.
You could pick one particular phase of the wave and it would appear to travel at the phase velocity.
The phase velocity is given in terms of the wave's angular frequency ω and wave vector k by

15. Group velocity where: vg is the group velocity;
Group velocity of a wave is the velocity with which the variations in the shape of the wave's amplitude (known as the modulation or envelope of the wave) propagate through space.
The group velocity is defined by the equation
where:
vg is the group velocity;
ω is the wave's angular frequency;
k is the wave number.
The function ω(k), which gives ω as a function of k, is known as the dispersion relation.

16. Velocity of De-Broglie Waves-Phase velocity
Moving material particle-De broglie said a wave is associated which travel with the same velocity as that of the wave.
Let De-broglie wave velocity is W, so Vp or W=

17. Phase Velocity - continued
we know that
=h/mv
From Planck`s Law
E = h
From mass energy relation
E = mc2 2

18. Phase Velocity - continued
h = mc2  = mc2/h
W =  
Therefore
W= mc2/h X h/mv
W = C2/v
So Particle velocity must be less than the velocity of light W > c
We can say that De-Broglie waves must travel faster than the velocity of light.

19. Group Velocity=Particle velocity

20. Group Velocity=Particle velocity.

21. De Broglie waves for massive particles

26. Scheme

27. The group velocity is the velocity of a pulse of light, that is, of its irradiance.
While we derived the group velocity using two frequencies, think of it as occurring at a given frequency, the center frequency of a pulsed wave. It’s the velocity of the pulse.
When vg = vf, the pulse propagates at the same velocity as the carrier wave (i.e., as the phase fronts): z
This rarely occurs, however.

28. When the group and phase velocities are different…
More generally, vg ≠ vf, and the carrier wave (phase fronts) propagates at the phase velocity, and the pulse (irradiance) propagates at the group velocity (usually slower).
The carrier wave:
The envelope (irradiance):
Now we must multiply together these two quantities.

29. Differences in speed cause spreading or dispersion of wave packets

30. The group velocity is the speed of the wavepacket
The phase velocity is the speed of the individual waves
Phase velocity = Group Velocity
The entire waveform—the component waves and their envelope—moves as one. non-dispersive wave.
Phase velocity = -Group Velocity
The envelope moves in the opposite direction of the component waves.
Phase velocity > Group Velocity
The component waves move more quickly than the envelope.
Phase velocity < Group Velocity
The component waves move more slowly than the envelope.
Group Velocity = 0
The envelope is stationary while the component waves move through it.
Phase velocity = 0
Now only the envelope moves over stationary component waves.

31. Relation Between Group Velocity and Phase Velocity

33. This gives us the relation between Phase velocity and Group velocity

34. This gives us the relation between Phase velocity and Group velocity

35. Thanks

36. Phase Velocity for a massive particle and massless Particle
phase velocity does not describe particle motion

37. De Broglie Waves -Photons.