# Speed of light through a medium

First off, please do not direct me to:

Does light slow down because of the absorption and re-emission of photons by electrons, and it appears the light is slowing down because of the slight delay this takes? but in between absorption and re-emission light is traveling at C?

Does light slow down because when it interacts with materials, those materials create electromagnetic waves that interfere with the photons slowing them down?

Or what is exactly going on here? I just need a very basic and exact answer to this question:

Do photons of light slow down through medium? and why?

THANKS!

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#### ZapperZ

Staff Emeritus
2018 Award
The apparent slowdown of the speed of light is typically a measure of the GROUP VELOCITY of light, i.e. this is the classical wave property.

You need to sit down and figure out what "speed" is exactly measured when people cite the number for the "speed of light". In a medium, it is always the group velocity, NOT the speed of photons! The speed of photons has been discussed already in the FAQ entry.

Zz.

#### Bill_K

Refraction is a collective phenomenon, meaning not just many atoms but also many photons. If you can identify an individual photon, it travels at c. Period. There is no such thing as a slowly moving photon.

If there are enough photons present to be considered as a classical electromagnetic wave, here's what happens. The time-varying E field of the wave produces a time-varying polarization in the medium. Topic 4 in the FAQ mentions one aspect of this, namely lattice vibrations or phonons. In the case of an ionic solid the atoms will be slightly displaced from their equilibrium positions. Individual atoms can also be polarized. (This does not mean "raised to an excited state". Solve the Schrodinger equation for an atom in a uniform electric field. You will find the electron cloud slightly displaced, and the energy slightly changed.)

These effects becomes especially clear if you measure the refractive index of a substance over a wide range of frequencies. You will find certain frequency ranges where the index changes rapidly. Typically they are in the infrared and the ultraviolet, corresponding to resonant frequencies of the material, where the induced polarization is large.

#### Meir Achuz

Homework Helper
Gold Member
The slowing of light is a collective phenomena of the molecules in the matter. The wave length (~5000 Angstroms) of a single photon spreads out to cover a huge number of molecules (about 1 Angstrom apart), so even a single photon's motion is covered by the usual classical EM equations.

A model of absorption and re-emission of photons is not appropriate until the wave length becomes of the order of an Angstrom.

#### Drakkith

Staff Emeritus
2018 Award
Where's the FAQ for this at again? Is it in a different forum?

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