Light in a medium - Question about the FAQ

[bubblewrap]

In the FAQ section 'Do Photons Move Slower in a Solid Medium?', it says that light is slower in a medium when its frequency is beyond the phonon spectrum. So do lights that have frequency within that spectrum move at the speed of c, not being slowed down by the medium?

 

[ZapperZ]

In the FAQ section 'Do Photons Move Slower in a Solid Medium?', it says that light is slower in a medium when its frequency is beyond the phonon spectrum. So do lights that have frequency within that spectrum move at the speed of c, not being slowed down by the medium?

No. There can be several outcomes for this depending on the nature of the material. The simplest being that the light tends to be absorbed and its energy dissipated via heat, i.e the material is opaque to this light. This is because there are vibrational modes of the material that correspond to the same frequency.

Zz.

 

[DrDu]

Light is almost always slower than the speed of light in vacuum, also above the phonon spectrum. Especially, for light in the visible range, the interaction with electronic excitations in the ultraviolet is much more important than the interaction with phonons in the IR.