# Speed of light through glass

In nearly every physics textbook I've encountered, it is always claimed in the optics section of the textbook that light travels slower through glass, water, diamond, etc. The speed of light in vacuum is divided by the material's index of refraction in order to calculate the "new speed" through the material.

Now, this idea of light slowing down perplexes me because it seems contrary to SR's postulate that the speed of light is constant and is the same for all observers. This contradiction is never explained in textbooks, yet there must be some logical reason for why light seems to slow down.

Would it be correct/accurate to say that as light travels through a transparent medium, it only seems to be slowing down because of absorption and re-emission of photons from atom to atom inside the medium? Each absorption/re-emission results in a time delay which creates the illusion of slower light?

Janus
Staff Emeritus
Gold Member
Dorje said:
Would it be correct/accurate to say that as light travels through a transparent medium, it only seems to be slowing down because of absorption and re-emission of photons from atom to atom inside the medium? Each absorption/re-emission results in a time delay which creates the illusion of slower light?
You've got it.

Though if you read SR carefully, you will note that Einstein says that it is the Speed of light in a vacuum is constant for all observers. Thus when you see c it refers specifically to the speed of light in a vacuum only.

What if someone lived in a transparent medium with an exceptionally large refraction index? What kind of implications would it have on their sense of time?

Janitor
Chen,

They would probably encounter lots of Cerenkov radiation, which results from a charged particle moving faster than speed c/n in a medium.

I read somewhere that when light enters a material of a different index, it should be thought of (classically, I suppose) as changing in wavelength but not in frequency, such that the product of wavelength and frequency inside the medium gives the speed of light inside the medium. Maybe someone here can expound on this?

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Well, assuming they don't move faster than light, just very close to its speed. Thanks for the information though!

Hurkyl
Staff Emeritus
Gold Member
Don't quote me on this but I understand that even in the classical model of an electromagnetic wave, if you go into more detail, it turns out the leading edge of the wave does actually travel through a medium at c, but this edge is very faint; the wave gets smeared out as it goes through the medium, so the bulk of the energy lags behind the leading edge, thus travelling at a fraction of c.

Janitor
Fascinating, Hurkyl.

And since you say "even the classical model..." I wonder if there are non-e.m. examples/analogues of what you are talking about. Sound waves, or what have you.

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I still say the model of absorption and re-radiation is totally wrong - every photon of whatever wavelength must be absorbed and released in exactly the same direction based upon a lot of speculation as to how long each photon is held captive etc - and there is no explanation of how "long wavelengths" get absorbed by a single atom ...and re-radiated. So here is a thought:

When sunlight traverses the earths atmosphere, the electric field interacts with the atoms - this interaction involves forces - if the electric field in the photon sets the electric charges into vibration - - the forces exerted on the charges in the atom must be balanced by inertial reactionary forces that act upon the photon - - the higher the frequency of the light, the greater the interaction with the atom since higher frequences are closer to the natural uv frequency of the electric charges in the atom - so the high frequency photons are slowed to a greater degree, as evidenced by the experimental fact that blue light is slowed more than red.

Pythagorean
Gold Member
Sorry if the necropost offends, I've been thinking about this, myself. It actually originated from thoughts on the quantization of space (or, spacetime if you like), and how the permitivity and permeability of substances might account for that.

In that regard, I was kind of sad when I realized that I think yogi is wrong:

I still say the model of absorption and re-radiation is totally wrong - every photon of whatever wavelength must be absorbed and released in exactly the same direction based upon a lot of speculation as to how long each photon is held captive etc - and there is no explanation of how "long wavelengths" get absorbed by a single atom ...and re-radiated. So here is a thought:
This may be me of a solid state topic really, but your concerns have already been addressed if I'm interpreting them correctly.

From what I remember from my solid state class, most materials actually do absorb and release randomly depending on their qualities (actually, in solid state view, it's just the molecular structure, since that accounts for the color and density of the object. More importantly though, is the shape of the structure).

For instance, an amorphous rock, the standard kind. Solid gray. They have no symmetric shape to their molecular structure. It's a hodgepodge of domains, a homogeneous mixture of molecules. So when light is shined on it, it mostly absorbs the photons, then emits them in (practically) all directions, as heat. That's why (especially darker) rocks will be warm after sitting in the sun.

If we go to the opposite side of the "solid state spectrum" (I made that up!) then we have nearly perfect crystal lattice structures. These structures have excellent, systematic light handling properties. Their structures are (nearly) perfect little three dimensional structures that are repeated over and over again some 10^23 times to make the chunk of crystal you would see. It is this structure (including it's shape and the properties of the atoms that make it up) that allow for excellent "light conduction"

Of course, no crystal is perfect I'm told, they all have tiny flaws, as far as transmission is concerned. That is, some absorption will (generally) occur even as light passes through, resulting in power loss, which will (generally) be radiated as heat.

What I wonder is... do the infrared photons contain the heat? Do I feel heat as my body absorbs infrared photons? Or is heat a separate thing that is emitted with infrared photons?

What I wonder is... do the infrared photons contain the heat? Do I feel heat as my body absorbs infrared photons? Or is heat a separate thing that is emitted with infrared photons?
Sorry to sound like a jerk, but how do you not know this! From your post you sound like you have an in depth knowledge of structures of solids but no understanding of radiation!

Heat is divided into Conduction, Convection and Radiation. Conduction as in the vibration of molecules in a substance (heat) that can only be transferred through other objects by contact, convection in the transfer of vibrational heat through a liquid or gas and radiation through photons (of infrared).

When you have a 'space heater' on you're generally feeling the radiative infra red photons striking your skin. These photons induce vibrations in the atoms thus increasing their temperature.

Anyway back on topic, ive never really though about it but why is there a time lag on the absorption/emission of an atom that causes this effect? One would assume the process would be near instantaneous, or for that matter should I ask, what's going on inside the electron that takes time for it to re-emit?

ZapperZ
Staff Emeritus
First of all, this is a rather old thread that probably was one of the impetus on why we have an entry on this issue in our FAQ. So if people haven't look at the FAQ in the General Physics forum, it might be best to start there first. The "doubt" regarding absorption of "long wavelengths" by solids has been sufficiently addressed many times already, and especially in Solid State physics texts.

Sorry to sound like a jerk, but how do you not know this! From your post you sound like you have an in depth knowledge of structures of solids but no understanding of radiation!

Heat is divided into Conduction, Convection and Radiation. Conduction as in the vibration of molecules in a substance (heat) that can only be transferred through other objects by contact, convection in the transfer of vibrational heat through a liquid or gas and radiation through photons (of infrared).

When you have a 'space heater' on you're generally feeling the radiative infra red photons striking your skin. These photons induce vibrations in the atoms thus increasing their temperature.

Anyway back on topic, ive never really though about it but why is there a time lag on the absorption/emission of an atom that causes this effect? One would assume the process would be near instantaneous, or for that matter should I ask, what's going on inside the electron that takes time for it to re-emit?
Secondly, if you want to look at it naively, one can think of the finite, non-zero mass of not only the electrons, but also the ions in the lattice. The ions making up the lattice, even in a non-crystalline material, will require some time to respond to the external E-field provided by the incoming light. So that can easily provide the delay if the oscillating ions, in turn, re-emit the light.

One can then expect that materials that are made up of heavier elements will have a higher index of refraction. This tends to be a general trend for most material, but note that there are many exception to this rule. This is because the refractive index is also affected by the lattice geometry (example: diamond versus graphite) and bonding configuration that make up the solid. So it isn't that simple.

Zz.

Pythagorean
Gold Member
Sorry to sound like a jerk, but how do you not know this! From your post you sound like you have an in depth knowledge of structures of solids but no understanding of radiation!

Heat is divided into Conduction, Convection and Radiation. Conduction as in the vibration of molecules in a substance (heat) that can only be transferred through other objects by contact, convection in the transfer of vibrational heat through a liquid or gas and radiation through photons (of infrared).

When you have a 'space heater' on you're generally feeling the radiative infra red photons striking your skin. These photons induce vibrations in the atoms thus increasing their temperature.
I understand that it can work this way (an atom absorbs a photon and raises it's energy level) but why does this just have to be infrared photons? Why are they the ones associated with heat? Surely UV rays give off heat, don't they?

Along with this, we know heat has units of energy, but surely there's energy that's not heat (gravitational potential energy, I guess, for one, but potential energy may be an unsatisfactory example).

Depending on your heating system, it may be a purely convective process, but is convection really a modern phenomena? I've only learned convection in a classical sense, but if you think about it convection is really conduction by moving particles (or at least that's how it seems to me, my intuition could be off here).

So wouldn't this mean there's actually no infrared photons involved, as the phonons are the ones transferring heat in conduction (or... are they?).

Or... do the phonons interact with each other through photons, absorbing and emitting just like with electrons. Would this imply that all heat transfer is actually radiative?

p.s. you sound like a jerk ;)

Sorry to sound like a jerk, but how do you not know this! From your post you sound like you have an in depth knowledge of structures of solids but no understanding of radiation!

Heat is divided into Conduction, Convection and Radiation. Conduction as in the vibration of molecules in a substance (heat) that can only be transferred through other objects by contact, convection in the transfer of vibrational heat through a liquid or gas and radiation through photons (of infrared).
Yes, you do sound like a jerk :D
They are all the same heat. Whether it's two particles or two objects, or a photon and a particle colliding, that's just all about it - collision, transfer of energy.

JesseM
First of all, this is a rather old thread that probably was one of the impetus on why we have an entry on this issue in our FAQ. So if people haven't look at the FAQ in the General Physics forum, it might be best to start there first.

https://www.physicsforums.com/showpost.php?p=899393&postcount=4 [Broken]

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I understand that it can work this way (an atom absorbs a photon and raises it's energy level) but why does this just have to be infrared photons? Why are they the ones associated with heat? Surely UV rays give off heat, don't they?

Along with this, we know heat has units of energy, but surely there's energy that's not heat (gravitational potential energy, I guess, for one, but potential energy may be an unsatisfactory example).
Yes, every form of electromagnetic radiation causes heat in matter. Looking at Black Body radiation, temperatures that we usually experience in day to day life mostly give off infra red radiation (heat an object up more, it starts to enter the visible spectrum).

Depending on your heating system, it may be a purely convective process, but is convection really a modern phenomena? I've only learned convection in a classical sense, but if you think about it convection is really conduction by moving particles (or at least that's how it seems to me, my intuition could be off here).

So wouldn't this mean there's actually no infrared photons involved, as the phonons are the ones transferring heat in conduction (or... are they?).

Or... do the phonons interact with each other through photons, absorbing and emitting just like with electrons. Would this imply that all heat transfer is actually radiative?

p.s. you sound like a jerk ;)
Well as far as I know all interactions of matter are by photons. A hot object constantly radiates infrared, however on the inside it is reabsorbed by its neighbouring particles so the only real loss of heat comes from the surface. From conduction since one object would be cooler than the other, the infrared radiation is absorbed by it thus being transferred and spread around the objects.

Found a fairly good explanation of what happens to a photon in matter:

In a material, photons couple to the excitations of the medium and behave differently. These excitations can often be described as quasi-particles (such as phonons and excitons); that is, as quantized wave- or particle-like entities propagating though the matter. "Coupling" means here that photons can transform into these excitations (that is, the photon gets absorbed and medium excited, involving the creation of a quasi-particle) and vice versa (the quasi-particle transforms back into a photon, or the medium relaxes by re-emitting the energy as a photon). However, as these transformations are only possibilities, they are not bound to happen and what actually propagates through the medium is a polariton; that is, a quantum-mechanical superposition of the energy quantum being a photon and of it being one of the quasi-particle matter excitations.
http://www.chemistrydaily.com/chemistry/Photon

I realize this post is a bit old, but Im a college student in physics and we actually went over this in a lab today. The analogy given is that youre in a car that can only go two speeds: 100mph and 0mph. When driving through the city, every stop sign, every stop light, every pedestrian and every cat, you instantaneously go from 100-0 then back to 100, causing the time it takes to go through the city to be much longer than if there were no obstacles. This analogy was related to the photons moving through a more dense medium, every time a photon encounters an obstacle, it instantaneously stops and continues (absorption-emission) at the speed of light, causing the speed of light to decrease while in a medium.. Well, not technically the speed of light, but the speed light is capable of traveling through a medium.

I realize this isn't necessarily as technical and extremely detailed as other responses, but its the easiest analogy to wrap your head around the idea that the speed of light can be significantly slower in glass.

JesseM
I realize this post is a bit old, but Im a college student in physics and we actually went over this in a lab today. The analogy given is that youre in a car that can only go two speeds: 100mph and 0mph. When driving through the city, every stop sign, every stop light, every pedestrian and every cat, you instantaneously go from 100-0 then back to 100, causing the time it takes to go through the city to be much longer than if there were no obstacles. This analogy was related to the photons moving through a more dense medium, every time a photon encounters an obstacle, it instantaneously stops and continues (absorption-emission) at the speed of light, causing the speed of light to decrease while in a medium.. Well, not technically the speed of light, but the speed light is capable of traveling through a medium.
That's right, the only subtlety is that the "obstacles" the photons are being absorbed and reemitted by are not actually individual atoms in the medium as one might expect, instead in some weird quantum way the photon is actually being absorbed and reemitted by the lattice of atoms as a whole...see ZapperZ's post #4 on this thread.

I think the speed of light is always the same and the "slowing down" of light is just another way to say the effective speed of light - for practical purposes you think of it as slowing down. I've read many of the technical explanations of the physics of what be happening, but approaching it from another angle, if the light were truly slowed, then when it left the glass it should remain at that same slowed speed unless some force acted on it to speed it up again right ?

So theoretically if i put up a long row of glass blocks (like a long row of dominos), the light should get slower each time it hits a new glass block until it going very slow and you could see it move. I think what happens though is that when it leaves the glass, the light is back at the normal speed of light. So I really think the "slowed" speed is just a net effect.

I think the speed of light is always the same and the "slowing down" of light is just another way to say the effective speed of light - for practical purposes you think of it as slowing down. I've read many of the technical explanations of the physics of what be happening, but approaching it from another angle, if the light were truly slowed, then when it left the glass it should remain at that same slowed speed unless some force acted on it to speed it up again right ?

So theoretically if i put up a long row of glass blocks (like a long row of dominos), the light should get slower each time it hits a new glass block until it going very slow and you could see it move. I think what happens though is that when it leaves the glass, the light is back at the normal speed of light. So I really think the "slowed" speed is just a net effect.
Yes, that's why the time delay hypothesis due to interaction with atoms is really the only feasible explanation. All proposed models boil down to that common idea.