# Speed of electromagnetic wave in matter

1. Jan 14, 2010

### Julle

I've been looking all over the web for this stuff and I simply can't find anything that gives a proper explanation.

What determines the speed of say, radio waves, in a medium? I know electromagnetic waves travel at the speed of light and as such I guess I can use the refractive index to figure it out, but what of materials where a refractive index isn't at hand?

A few examples question to get my meaning across a bit better:

How fast does radio waves propagate in fresh water? Do they propagate at the same speed in sea water? What if I increase the salinity of the sea water, does that change the speed?

What about something like a dense vs a not so dense plastic?

Can it be assumed that a more dense material also slows down the radio waves more or is it something completely different that affects the speed?

2. Jan 14, 2010

### mgb_phys

Electromagnetic waves (eg light) always travel at the speed of light.
In a material photons travel at the speed of light until they hit an atom, they are then absorbed and excite that atom, a very short time later that atom de-excites and re-emits the photon.
It is this delay that produces the slow down of the speed of light in a material = the refractive index.

The refractive index isn't directly related to density in different material.
Although in many crystals and glasses it's a good approximation because density tells you how many atoms per cm^3 and so how many atoms a photon will interact with in it's passage.

The refractive index of water is about 1.33 but the salinity and temperature of water does effect the refractive index.
You can see this diving, a stream of fresh water from a spring will distort the image you see through it.

salinity
(gm/kg) increase in n(w) example
--------------------------------------------------------
5 0.00097 northern Baltic Sea
10 0.00194
15 0.00290
20 0.00386 bight of Biafra
25 0.00482
30 0.00577
35 0.00673 Atlantic surface
40 0.00769 northern Red Sea

3. Jan 14, 2010

### Naty1

A FAQ at the beginning of this forum describes a DIFFERENT process and claims the above explanation incorrect:

See that FAQ for a more complete explanation.

4. Jan 14, 2010

### mgb_phys

Yes - it's a gross over simplification to say it is simply absorbed and re-emitted by an atom.
It interacts with the various fields formed by the arrangements of the atoms, the lattice and various phonon states.

But it seemed a reasonable model for this level of answer - I should probably have said 'interacts with the material' rather than hits an atom

5. Jan 14, 2010

### nasu

The index of refraction (and the speed in material) depends on frequency.
So you cannot assume that the index of refraction for visible light (about 1.33) is valid for any electromagnetic waves.
In order to find an answer to your question you need first to make more precise (what kind of radio waves, what frequency range) and then look for info, if available.
Dispersion curves for various materials (speed of em waves versus frequency) can be found.
For sure you'll find them for water, maybe even for sea water.