# Why does the speed of light vary? **URL:** \ **Category:** Factual Questions\ **Created:** [February 6, 2003, 12:07pm UTC](https://boards.straightdope.com/t/why-does-the-speed-of-light-vary/153180 "2003-02-06T12:07:44Z")\ **Posts on this page:** 18\ **Page:** 1
**Author:** ![starman](https://avatars.discourse-cdn.com/v4/letter/s/8c91f0/32.png) [@starman](https://boards.straightdope.com/u/starman)\ **Post date:** [February 6, 2003, 12:07pm UTC](https://boards.straightdope.com/t/why-does-the-speed-of-light-vary/153180/1 "2003-02-06T12:07:44Z")
Why does the speed of light vary with the density of the medium through which light is travelling? That is, do photons interact with the particles in the medium they are passing through, and if so, then in what fashion? -Oli ---
**Author:** ![Mangetout](https://sea3.discourse-cdn.com/straightdope/user_avatar/boards.straightdope.com/mangetout/32/19_2.png) [@Mangetout](https://boards.straightdope.com/u/Mangetout)\ **Post date:** [February 6, 2003, 12:20pm UTC](https://boards.straightdope.com/t/why-does-the-speed-of-light-vary/153180/2 "2003-02-06T12:20:46Z")
I believe that the photons are (occasionally) absorbed and re-emitted as they pass through. ---
**Author:** ![starman](https://avatars.discourse-cdn.com/v4/letter/s/8c91f0/32.png) [@starman](https://boards.straightdope.com/u/starman)\ **Post date:** [February 6, 2003, 12:28pm UTC](https://boards.straightdope.com/t/why-does-the-speed-of-light-vary/153180/3 "2003-02-06T12:28:25Z")
That was my assumption also. OK, so I take it that there is a certain delay between absorbtion and re-emission then. But why does the speed of light vary with the density of the medium then, and not the electron structure of the molecules in the medium? Thanks for the reply, -Oli ---
**Author:** ![Mangetout](https://sea3.discourse-cdn.com/straightdope/user_avatar/boards.straightdope.com/mangetout/32/19_2.png) [@Mangetout](https://boards.straightdope.com/u/Mangetout)\ **Post date:** [February 6, 2003, 12:35pm UTC](https://boards.straightdope.com/t/why-does-the-speed-of-light-vary/153180/4 "2003-02-06T12:35:14Z")
IANAPhysicist, but I think it’s that the less dense the material, the more likely that the photon will just zip through without hitting anything. ---
**Author:** ![Desmostylus](https://avatars.discourse-cdn.com/v4/letter/d/c57346/32.png) [@Desmostylus](https://boards.straightdope.com/u/Desmostylus)\ **Post date:** [February 6, 2003, 12:44pm UTC](https://boards.straightdope.com/t/why-does-the-speed-of-light-vary/153180/5 "2003-02-06T12:44:15Z")
The speed of light in a medium varies inversely with the square root of the relative permittivity. That is, v = c / root(e[sub]r[/sub]) ---
**Author:** ![starman](https://avatars.discourse-cdn.com/v4/letter/s/8c91f0/32.png) [@starman](https://boards.straightdope.com/u/starman)\ **Post date:** [February 6, 2003, 12:45pm UTC](https://boards.straightdope.com/t/why-does-the-speed-of-light-vary/153180/6 "2003-02-06T12:45:16Z")
Yes, but I should have thought the lectron shell structure of the molecules in the medium would have also been important. That is, normally you wouldn’t expect air, for example to absorb and re emit light in visible wavelengths. Additionaly, why does light retain directionality when passing through a medium? That is, if atoms are responsible for absorbing and re-emitting the light, why is the light all re-emitted in the same direction? Normally, for example in atomic absorbtion spectroscopy, an atom would be expected to absorb light and re-emit it in an essentially random direction. -Oli ---
**Author:** ![starman](https://avatars.discourse-cdn.com/v4/letter/s/8c91f0/32.png) [@starman](https://boards.straightdope.com/u/starman)\ **Post date:** [February 6, 2003, 12:56pm UTC](https://boards.straightdope.com/t/why-does-the-speed-of-light-vary/153180/7 "2003-02-06T12:56:28Z")
Ah, make that electron shell structure. -Oli ---
**Author:** ![Desmostylus](https://avatars.discourse-cdn.com/v4/letter/d/c57346/32.png) [@Desmostylus](https://boards.straightdope.com/u/Desmostylus)\ **Post date:** [February 6, 2003, 12:56pm UTC](https://boards.straightdope.com/t/why-does-the-speed-of-light-vary/153180/8 "2003-02-06T12:56:42Z")
Photons aren’t absorbed and re-emitted. If that happened, the material would be reflective, not transparent. The electric charge properties of the medium _are_ important, and in particular, the one I pointed to, the relative permittivity. Relative premittivity is also called dielectric constant. ---
**Author:** ![starman](https://avatars.discourse-cdn.com/v4/letter/s/8c91f0/32.png) [@starman](https://boards.straightdope.com/u/starman)\ **Post date:** [February 6, 2003, 12:59pm UTC](https://boards.straightdope.com/t/why-does-the-speed-of-light-vary/153180/9 "2003-02-06T12:59:28Z")
Thanks for your reply. I am currently doing HS Chem and Phys, so could you explain what the dielectric constant and the relative permittivity of a substance are, in physical terms? Thanks, -Oli ---
**Author:** ![Desmostylus](https://avatars.discourse-cdn.com/v4/letter/d/c57346/32.png) [@Desmostylus](https://boards.straightdope.com/u/Desmostylus)\ **Post date:** [February 6, 2003, 1:06pm UTC](https://boards.straightdope.com/t/why-does-the-speed-of-light-vary/153180/10 "2003-02-06T13:06:17Z")
Simple explanation: Polar molecules in a medium increase the medium’s dielectric constant. [See here for some information.](http://hyperphysics.phy-astr.gsu.edu/hbase/electric/dielec.html) ---
**Author:** ![starman](https://avatars.discourse-cdn.com/v4/letter/s/8c91f0/32.png) [@starman](https://boards.straightdope.com/u/starman)\ **Post date:** [February 6, 2003, 1:10pm UTC](https://boards.straightdope.com/t/why-does-the-speed-of-light-vary/153180/11 "2003-02-06T13:10:07Z")
That is very interesting, but I still don’t understand why a change in the dielectric constant means a change of the speed of light in that medium. Thanks for your patience, -Oli ---
**Author:** ![Desmostylus](https://avatars.discourse-cdn.com/v4/letter/d/c57346/32.png) [@Desmostylus](https://boards.straightdope.com/u/Desmostylus)\ **Post date:** [February 6, 2003, 1:17pm UTC](https://boards.straightdope.com/t/why-does-the-speed-of-light-vary/153180/12 "2003-02-06T13:17:33Z")
Further explanation will need to get into Maxwell’s equations. Putting it into simple terms is reasonably difficult. I’m not up to it right now. It’s a good question, though. Hopefully someone more patient will come along later. ---
**Author:** ![Mangetout](https://sea3.discourse-cdn.com/straightdope/user_avatar/boards.straightdope.com/mangetout/32/19_2.png) [@Mangetout](https://boards.straightdope.com/u/Mangetout)\ **Post date:** [February 6, 2003, 1:18pm UTC](https://boards.straightdope.com/t/why-does-the-speed-of-light-vary/153180/13 "2003-02-06T13:18:19Z")
I don’t know where I got my absorption/re-emission idea from (I thought it was from another thread, but I can’t find it) - time to wind my neck in and leave it to the experts (sorry). ---
**Author:** ![starman](https://avatars.discourse-cdn.com/v4/letter/s/8c91f0/32.png) [@starman](https://boards.straightdope.com/u/starman)\ **Post date:** [February 6, 2003, 1:26pm UTC](https://boards.straightdope.com/t/why-does-the-speed-of-light-vary/153180/14 "2003-02-06T13:26:16Z")
So, as far as I understand, light does not interact with electrons in the medium directly. So what does the light interact with? What forces are acting upon the photons? I apologise for the clumsy wording. -Oli ---
**Author:** ![Desmostylus](https://avatars.discourse-cdn.com/v4/letter/d/c57346/32.png) [@Desmostylus](https://boards.straightdope.com/u/Desmostylus)\ **Post date:** [February 6, 2003, 1:37pm UTC](https://boards.straightdope.com/t/why-does-the-speed-of-light-vary/153180/15 "2003-02-06T13:37:38Z")
Consider the wave/particle duality. In this case, it helps to think of light in terms of waves rather than particles (photons). Light is an electromagnetic wave, a wave having an electric component and a magnetic component. The electric and magnetic properties of the medium through which the wave propagates affect the speed of propagation. ---
**Author:** ![starman](https://avatars.discourse-cdn.com/v4/letter/s/8c91f0/32.png) [@starman](https://boards.straightdope.com/u/starman)\ **Post date:** [February 6, 2003, 1:44pm UTC](https://boards.straightdope.com/t/why-does-the-speed-of-light-vary/153180/16 "2003-02-06T13:44:11Z")
Ah. I think I am closer to understanding now. Thanks for your help. I’ll see what other stuff I can dig up on the net. Regards, -Oli ---
**Author:** ![ZenBeam](https://avatars.discourse-cdn.com/v4/letter/z/3ab097/32.png) [@ZenBeam](https://boards.straightdope.com/u/ZenBeam)\ **Post date:** [February 6, 2003, 5:29pm UTC](https://boards.straightdope.com/t/why-does-the-speed-of-light-vary/153180/17 "2003-02-06T17:29:45Z")
Absorption and reemission isn’t the right way to think of it. If you put even a non-polar atom or molecule in a constant electric field, you will get some charge separation, as the electrons are pulled one way, and the nucleus the other. In the case of light propagating through the material, this charge separation follows the electric field, with the maximum separation occuring when the electric field is at its maximum. This is a little dipole antenna, which radiates at the same frequency as the wave. The field radiated by this dipole adds 90 degrees out of phase with the primary wave, and the total field is then delayed a bit compared with the primary field. This leads to the slowing of the wave. The dipole usually isn’t _exactly_ 90 degrees out of phase with the primary wave, and this small difference is what causes loss as a wave travels through the material. With polar molecules, such as water, the effect can be much larger than for non-polar molecules, if the mocule has time to react to the wave. For water, for example, the dielectric constant is very large (around 90) for frequencies below about 10 GHz. Above that frequency, the dielectric constant drops off, because the molecule doesn’t have time to react and rotate to allign with the primary field. ---
**Author:** ![Ring](https://avatars.discourse-cdn.com/v4/letter/r/6a8cbe/32.png) [@Ring](https://boards.straightdope.com/u/Ring)\ **Post date:** [February 6, 2003, 6:25pm UTC](https://boards.straightdope.com/t/why-does-the-speed-of-light-vary/153180/18 "2003-02-06T18:25:34Z")
**ZenBeam’s** wave explanation is excellent. Below is the particle explanation. I’ve written this explanation a number of times and I think it is one of mine, but I’m not sure so I may be plagiarizing. In any case it’s correct. > [@](#): > > If the light travelling through a medium has a frequency which doesn’t correspond to any of the allowed energy transitions of the atoms in the medium, it is not “truly” absorbed by the atoms. When a photon interacts with an atom in the medium it momentarily forms a combined atom-photon energy state, sometimes called a “dressed state”. Another way of looking at this is that the atom is excited to a “virtual” energy state. This virtual state is > not an “allowed” energy state of the atom, so the atom almost immediately re-emits the photon. > > Virtual interactions take some time to happen, and this is what is responsible for the apparent slowing of light as it passes through a medium. A transparent medium, by the way, is one in which the incident light doesn’t correspond to any allowed atomic transition, making all the interactions virtual. > > Virtual energy levels are permitted by the Heisenberg uncertainty principle relating energy and time. A photon can be absorbed to a virtual energy level differing from an allowed level by an amount dE, provided that the atom does not stay in the virtual energy state for longer than time dt, where dE times dt is approximately equal to Planck’s constant. > > Consider glass. The apparent speed of blue light in glass is less than the speed of red light because the glass possesses allowed energy levels corresponding to transitions in the ultraviolet region of the spectrum. The difference in energy between the allowed transition energies and the blue light energy is less than the corresponding difference for the red light, meaning that the blue photons can be absorbed for longer times according to the uncertainty principle. The difference in apparent speeds is what allows a prism to split white light into its separate colors. > > In the case of virtual absorption and re-emission, since the photon is not “really” being absorbed by the atom its quantum state does not change. This means that it retains its original frequency, polarization and direction of travel when it is re-emitted.