Group Velocity and Phase Velocity

In summary, the conversation discusses finding the group and phase velocities of an extraordinary wave in a uniaxial crystal. The magnitude of the group velocity can be found using the definition of group velocity and the equation for the wave normal surface. The angle between the phase and group velocities (and field vectors) can be derived and it is shown to be zero at certain angles. The angle at which it is maximized can be calculated using the given values for the refractive indices of quartz. The maximum angular separation occurs at 45 degrees and is proportional to the difference between the two refractive indices. Further information is needed to provide more specific help.
  • #1
Joystar77
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I have no idea how to do this or where to start. Can someone please help me?

Problem 4.4- Suppose n o and n e are given. In (a) you only need to find the magnitude of the group velocity. Problem #2 in HW 10 may be helpful. You can also directly use the definition of group velocity, i.e., v g = triangle k w (k), taking into account the equation of the wave normal surface.

4.4- Group Velocity and Phase Velocity

a.) Derive an expression for the group velocity of the extraordinary wave in a uniaxial crystal as a function of the polar angle 0 of the propagation vector.

b.) Derive an expression for the angle a between the phase velocity and the group velocity. This angle is also the angle between the field vectors E and D.

c.) Show that a = 0 when 0 = 0, ½ pi. Find the angle at which a is maximized and obtain an expression for a max. Calculate this angle a max for quartz with n o = 1.554, n e = 1.553.

d.) Show that for no or ne, the maximum angular separation a max occurs at 0 = 45 degrees; show that a max is proportional to [ n o – n e].
 
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  • #2
I'm afraid no one is going to be able to help you without more information. Certainly no one here knows what "problem #2 in HW 10" is!

What are you using as the definitions of "phase velocity" and "group velocity"?
 

FAQ: Group Velocity and Phase Velocity

What is the difference between group velocity and phase velocity?

Group velocity refers to the speed at which a wave packet (a group of waves) travels, while phase velocity refers to the speed at which an individual wave travels. Group velocity takes into account the overall motion of the entire wave packet, while phase velocity only considers the motion of the individual waves within the packet.

How are group velocity and phase velocity related?

Group velocity and phase velocity are related by the dispersion relation, which describes how the frequency and wavelength of a wave are related. In most cases, the group velocity is equal to the phase velocity, but in certain situations, such as in dispersive media, they can be different.

What is the significance of group velocity and phase velocity?

Group velocity and phase velocity are important concepts in the study of wave phenomena. They help us understand how waves propagate and interact with different mediums. For example, in optics, the group velocity determines the speed at which an image or signal travels through a material, while the phase velocity is important in determining the angle of refraction as a wave passes through a boundary between two materials.

Can group velocity be greater than the speed of light?

No, according to the laws of physics, the group velocity of a wave cannot exceed the speed of light. This is because the group velocity is determined by the speed of the individual waves within the wave packet, and these waves cannot travel faster than the speed of light.

How does the medium affect the group and phase velocities of a wave?

The properties of the medium, such as its density, temperature, and composition, can affect the group and phase velocities of a wave. For example, in a medium with a high refractive index, such as water, both the group and phase velocities of light will be slower compared to a lower refractive index medium, such as air.

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