Physical origin of different phase velocities.

In summary: This is known as the classical optics model.In summary, the phase velocity of monochromatic light in a material can deviate from the speed of light in vacuum, which is constant according to special theory of relativity. This is due to the interactions between the photons and the electrons of the material, resulting in a shift of phase and varying velocities of different components of the wave-packet. This is known as the classical optics model, which contradicts the concept of constant speed of light in vacuum.
  • #1
Xian
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When monochromatic light propagates through a material other than a vacuum, we observe that its phase velocity has moved away from c. By relativity, we know that massless particles like the photon must move at the c however the pragmatics of classical optics ask of us to overlook this fact in favor of the practical model in which the speed of light in material can have variety. This are technically contradictory models and something has to give, and I'm 99% certain its classical optics.

So when it is said that the speed of light in a medium is some phase velocity [tex]v_p[/tex] what does this physically mean, and why the observed deviation from c?
 
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  • #2
The photons get scattered from the electrons of the material. Each scattering event leads to a shift of phase of the photons. On a coarse scale, this looks as if the phase of the photons either moves more rapidly or more slowly than in vacuum, leading to a higher or lower phase velocity.
 
  • #3
Light propagates as a wave-packet (mixture of different monochromatic frequencies); each frequency of oscillation interacts differently with the material (constituting its dispersion relation). This gives rise to various components of the wave-packet traveling with different velocities.

Special theory of relativity mentions that the speed of electromagnetic radiation in vacuum is constant = c.

M
 

Related to Physical origin of different phase velocities.

1. What is the physical origin of different phase velocities?

The physical origin of different phase velocities is related to the different speeds at which waves travel through different mediums. Phase velocity is the speed at which a specific point on a wave moves through space, while the group velocity is the speed at which the overall shape or "envelope" of the wave travels. These velocities can differ due to variations in the characteristics of the medium, such as density, temperature, and composition.

2. How does the medium affect the phase velocity of a wave?

The phase velocity of a wave is affected by the properties of the medium through which it is traveling. In general, waves travel faster through denser materials, and slower through less dense materials. Additionally, the phase velocity can also be affected by other factors such as temperature, pressure, and the presence of magnetic or electric fields.

3. Can two waves with different phase velocities have the same group velocity?

Yes, it is possible for two waves with different phase velocities to have the same group velocity. This can occur when the phase velocities of the individual waves cancel each other out, resulting in a net group velocity that is the same for both waves. This phenomenon is known as phase matching and is commonly observed in nonlinear optics and other wave phenomena.

4. What are some examples of waves with different phase velocities?

Some examples of waves with different phase velocities include electromagnetic waves (such as light and radio waves) traveling through different materials, sound waves traveling through different mediums (such as air and water), and seismic waves (such as P-waves and S-waves) traveling through different layers of the Earth's interior.

5. How does dispersion relate to different phase velocities?

Dispersion is the phenomenon where different frequencies of a wave travel at different speeds through a medium. This is related to different phase velocities because the frequency of a wave is directly proportional to its phase velocity. Therefore, materials with higher dispersion will have a greater difference in phase velocities for different frequencies of waves, resulting in a more spread out or "dispersed" wave. This can be observed in phenomena such as rainbows and prisms where light is dispersed into its component colors due to differences in phase velocity for each color.

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