# Physical origin of different phase velocities.

1. Apr 19, 2010

### Xian

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 $$v_p$$ what does this physically mean, and why the observed deviation from c?

2. Apr 20, 2010

### DrDu

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. Apr 28, 2010

### mavipranav

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 travelling with different velocities.

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

M