What causes light to slow down when it goes through media that aren't vacuums?

SOL

It's all in the title really. I was having this disscussion at lunch time and noone could offer a satisfactory explanation, maybe someone here could shed some light on the matter?

(sorry)

FISMA

Hi SOL,
The speed of light is a constant. Light does not speed up or slow down. Light goes the speed of light or it doesn't go at all.

Shame on any text book that says light slows down when it enters glass. Unfortunately, so many otherwise excellent collegiate level introductory Physics do so. Open up to the Optics chapters and you'll see.

What is going on is absorption and remission.

Basically, there is a collision where, under certain conditions, the old photon is absorbed (bye-bye). Then a new photon is re-emitted (welcome). Energy, charge, and nucleon number are all conserved.

This does not happen instantaneously, it takes a small amount of time. The more interactions, the more time it takes. This extra time is what causes us to think that light is going slower in the material when in fact, it never is.

Another good question is how does the light know to continue on in the same direction before and after the collision? True, angular momentum has to be conserved. However, I have always wondered why.

Check again later and I'll post more or again. There's a few things I need to re-read like fluorescence.

Slan

FISMA

OP,
It's been a while since I studied this stuff, hence, if I fall short of the mark, sorry. Professor Fink and Morbert are much more knowledgeable in the Quantum area and I am sure they will lend a hand.

So we have light, lots of photons heading from a vacuum into some kind of matter. Let's consider a single photon.

The single photon enters the matter and soon interacts with an electron.

Now there are a lot of interactions, I'll take a basic example.

A photon is coming in say from the left hand side and has an energy of E = hv. This photon interacts with an electron in [say] the kth orbital.

Bye bye photon, it is no more. Also, that electron becomes a photoelectron, not light, and leaves the atom with an energy E = hv-be, where be is the binding energy. An electron that gets kicked out of the atom due to photon excitation is a photoelectron.

Now there's a vacancy for an electron in the lowest energy level. Electrons love to be there so an electron from [say] the Mth orbital falls down - goes to the lower orbit.

However, this electron must give up some energy to get to the lower level. The energy released by this electron is fluorescent radiation and its energy is E = delta be.

Again, there are lot's of ways that light, visible or not, interacts with matter: compton scattering, microwave, visible, ultraviolet, x, and so on.

Have a look at this page
http://hyperphysics.phy-astr.gsu.edu/hbase/hframe.html

However, again, light/photons are always going the speed of light. During an interaction, there must be some finite time delay. Given the large number of interactions, these time delays add up and make it appear that light went through the medium slower than normal. That is not true.

I am very clear whenever I talk about this, especially during intro optics. I tell people that light never slows down.

I wish the texts would do the same. However, until my crusade to rid physics texts of the oh-so-awful word "moves" is accomplished, this one is on the back-burner.

SOL

While you might be right to say light doesn't slow down on the microscopic scale what I am really looking for is an explanation of why it slows down on the macroscopic scale.

We thought about the idea that it's an absorption/reemmision thing, however this explanation fails to deal with a number of problems:

1) The levels available for photon absorbtion through the above mechanism are quantised which means that it would affect certain wave lenghts more than others (also even with this some transitions are favoured), but, this doesn't really fit with the effect that you see in a prism

2) The quantum yield from flouresence in any system is quite a lot less than 100% which would make transparency very hard to achieve

3) This and other processes (vibrational/rotational levels) are all subject to release though other mechanisms (collsion, intersystem crossing leading to phosphoresence, internal conversion leading to other energy states...)) (this is kinda point 2 again)

4) Water wouldn't be transparent because the atom absorbing the photon is going to be moving/colliding and will have no way of emmiting the photon along the same path again...

5) This is more of a question, but surely you can get refraction at radio wave frequencies but there are no radiowave absorbtion levels of this nature?

There were other problems, but I can't remember what they were... but maybe you can address these?

Morbert

The slowing of light in a transparent medium is due to induced polarization currents in the medium. These induced currents cause (and are caused by) perturbations in the propagating electromagnetic field that collectively slow the propagation through the medium. Basically, if you sum up the perturbations (that each propagate at the speed of light), you get a collective propagation that's less than the speed of light.

http://ajp.aapt.org/resource/1/ajpias/v60/i4/p309_s1?isAuthorized=no

FISMA

Morbert wrote: »

The slowing of light in a transparent medium is due to induced polarization currents in the medium. These induced currents cause (and are caused by) perturbations in the propagating electromagnetic field that collectively slow the propagation through the medium. Basically, if you sum up the perturbations (that each propagate at the speed of light), you get a collective propagation that's less than the speed of light.

http://ajp.aapt.org/resource/1/ajpias/v60/i4/p309_s1?isAuthorized=no

Looks like a fun read, however, does anyone have access? If not, could you elaborate Morbert?

Morbert

FISMA wrote: »

Looks like a fun read, however, does anyone have access? If not, could you elaborate Morbert?

If you want the paper I can email it to you, though it's pretty convoluted.

Basically it says that an incoming wave will induce oscillating electric dipoles in the molecules. These dipoles, in turn, release electromagnetic waves, which induce more dipoles, which release more waves etc. When the original electromagnetic wave and these induced waves are all added up, the collective speed of propagation is slower than light.

SOL, from a quantum mechanical perspective, it is correct(-ish) to classify it as absorption and re-emission, but the electrons are not excited to higher real quantum states (which, as you say, are discrete). Instead, they are excited to virtual states (since real states are discrete and therefore not accessible unless the photon has a specific energy). The virtual state only last a small amount of time before reverting to the original state and sending the photon on its way, with the same energy and direction (since the excited state wasn't real), but slightly delayed . It is precisely because radio waves are bad at exciting electrons to higher real energy levels that make them good at passing through media.

Akrasia

I was always under the impression that light does not slow down when travelling through space, but time being relative, can mean that even while light is travelling at a constant speed, we may at certain times be travelling through time at a slower or faster speed relative to the photons of light elsewhere in space/time.

(this is seperate to the refraction experienced as light passes through different mediums)

Morbert

Akrasia wrote: »

I was always under the impression that light does not slow down when travelling through space, but time being relative, can mean that even while light is travelling at a constant speed, we may at certain times be travelling through time at a slower or faster speed relative to the photons of light elsewhere in space/time.

(this is seperate to the refraction experienced as light passes through different mediums)

The local speed of light relative to us is always c.

himnextdoor

It does seem confusing though. Suppose you have a source of light that emits at regular interval and a detector that can measure the time between emission and detection you could evaluate 'c'. If you then introduced a pane of glass between the emitter and detector then took the same measurement, wouldn't you arrive at the same value for 'c'?

Is it not the case that at a sub-atomic level, the pain of glass is comprised almost entirely of space; that the distances between the sub-atomic particles is many times greater than their radii? What is the likelihood then of a photon interacting with anything at all on its journey to the detector?

Absorption would be manifest in a loss of signal strength and would correspond to a 'heating up' of the medium through which the light has travelled. Such photons would be 'lost' to the detecter.

However, if you were to turn the pane of glass so as to increase the amount of time a photon is travelling through it, then you would see a change to the measurement of 'c'; photons would take slightly longer to arrive at the plane of detection. This is entirely the result of diffraction; the path of the light is bent and therefore it has to travel slightly further to get to the detector.

And this is what seems strange to me; a photon cannot interact with anything except in the case of a direct collision with an electron or something, at which point it ceases to exist. A photon's lot is to travel in a straight line at a constant velocity until it is absorbed. For a photon to have constant velocity it must be uninfluencable. If it can be turned then it can be slowed. And if it can be slowed, what would compel it to accelerate again? What force could bend the path of a photon?

Altering the angle of the glass in our experiment is enough to cause a significant bending of light but the amount of absorption through photon collisions is still insignigicant and anyway, any photons emitted as a result of absorption would not have the same energy or direction as the original ones did; they would be randomly scattered and would not interact with other photons eventually being dissipated as heat.

If photons are massless point particles that are unaffected by magnetism or electrostatic charge then what is it about changing the angle of the glass that causes them to change direction? Surely it is not the immense gravity of the electrons residing in the glass lattice that causes a stretching of space/time causing an apparent change of path?

It makes more sense to think of light in terms of waves. That way diffraction can be explained as the effect that particular sub-atomic structures have on the shape of any wave it encounters. The lattice structure of glass behaves as an energy/frequency dependent wave-guide.

Morbert

himnextdoor wrote: »

It does seem confusing though. Suppose you have a source of light that emits at regular interval and a detector that can measure the time between emission and detection you could evaluate 'c'. If you then introduced a pane of glass between the emitter and detector then took the same measurement, wouldn't you arrive at the same value for 'c'?

Is it not the case that at a sub-atomic level, the pain of glass is comprised almost entirely of space; that the distances between the sub-atomic particles is many times greater than their radii? What is the likelihood then of a photon interacting with anything at all on its journey to the detector?

Absorption would be manifest in a loss of signal strength and would correspond to a 'heating up' of the medium through which the light has travelled. Such photons would be 'lost' to the detecter.

My previous post describes the type of "absorption" responsible for transparency, and why such photons are not lost.

However, if you were to turn the pane of glass so as to increase the amount of time a photon is travelling through it, then you would see a change to the measurement of 'c'; photons would take slightly longer to arrive at the plane of detection. This is entirely the result of diffraction; the path of the light is bent and therefore it has to travel slightly further to get to the detector.

You wouldn't see a change in c by turning the pane of glass. Nor is it diffraction that you are describing. The bending of light as it moves from one medium to another is refraction.

And this is what seems strange to me; a photon cannot interact with anything except in the case of a direct collision with an electron or something, at which point it ceases to exist. A photon's lot is to travel in a straight line at a constant velocity until it is absorbed. For a photon to have constant velocity it must be uninfluencable. If it can be turned then it can be slowed. And if it can be slowed, what would compel it to accelerate again? What force could bend the path of a photon?

A photon can't travel at any speed other than the speed of light. It never needs to be accelerated because it never travels at any other speed.

Altering the angle of the glass in our experiment is enough to cause a significant bending of light but the amount of absorption through photon collisions is still insignigicant and anyway, any photons emitted as a result of absorption would not have the same energy or direction as the original ones did; they would be randomly scattered and would not interact with other photons eventually being dissipated as heat.

See my previous post. The electrons are not excited to real states. Instead, the electron and the photon are in a virtual state and quickly decay back to the original state, allowing the photon to continue travelling on its way.

If photons are massless point particles that are unaffected by magnetism or electrostatic charge then what is it about changing the angle of the glass that causes them to change direction? Surely it is not the immense gravity of the electrons residing in the glass lattice that causes a stretching of space/time causing an apparent change of path?

It makes more sense to think of light in terms of waves. That way diffraction can be explained as the effect that particular sub-atomic structures have on the shape of any wave it encounters. The lattice structure of glass behaves as an energy/frequency dependent wave-guide.

The state of a system of photons in quantum mechanics is represented by a wave. This wave is equivalent to the classical electromagnetic wave.

himnextdoor

My bad; refraction not diffraction.

Morbet:

But are you saying that when a photon hits one side of a transparent material, through a 'Newton's Cradle' type of effect, a photon with exactly the same characteristics is emitted on the other side; that an electro-magnetic wave carries the energy of the photon across the medium where it creates an exact replica of the original photon and emits it? That works for me, eardrums work the same way. I think it is reasonable to say that matter and energy appear as transducers to each other. My problem is the notion of actual particles of energy travelling through any medium at all; why.

Why can't the way you have described transparency be applied to space? If we can think of electromagnetic energy in terms of a shaking spring conducting energy in the direction of tension (which would account for the constant 'c') why have actual particles travelling through space?

Morbert

himnextdoor wrote: »

Why can't the way you have described transparency be applied to space? If we can think of electromagnetic energy in terms of a shaking spring conducting energy in the direction of tension (which would account for the constant 'c') why have actual particles travelling through space?

In Quantum Field Theory, particles are simply excitations of fields. A photon travelling through space is an electromagnetic wave, and waves of spacetime itself can propagatate and (in principle) be represented by graviton particles. Similarly, mechanical vibrations or plasma oscillations can be represented by quasiparticles like phonons or plasmons.

himnextdoor

Morbert wrote: »

In Quantum Field Theory, particles are simply excitations of fields. A photon travelling through space is an electromagnetic wave, and waves of spacetime itself can propagatate and (in principle) be represented by graviton particles. Similarly, mechanical vibrations or plasma oscillations can be represented by quasiparticles like phonons or plasmons.

LOL. I knew there existed some common ground.

The thing is though, what I can't quite ascertain is, does modern science accept the notion of an aether?

Morbert

himnextdoor wrote: »

LOL. I knew there existed some common ground.

The thing is though, what I can't quite ascertain is, does modern science accept the notion of an aether?

No. An aether, unlike a field, is not lorentz invariant.

http://en.wikipedia.org/wiki/Lorentz_invariance

himnextdoor

Morbert wrote: »

No. An aether, unlike a field, is not lorentz invariant.

http://en.wikipedia.org/wiki/Lorentz_invariance

Okay but I refer to the Michelson-Morley experiments (and others) which, at the time were trying to settle a two-way dispute regarding the nature of an aether which was presumed to exist and thus, in failing, sounded a death-knoll for the notion of an aether. But I think there could be a third angle to the dispute and a small rearranging of the standard method for the experiment could test for it.

Suppose that matter represents an area depleted of space and trying to equalise spacial pressure space 'pervades' the matter and in doing so provides energy to the system which is radiated off as EMR. This would cause a 'current' of space to bear down in all directions on the mass. The larger the mass, the larger the current.

If the aether were flowing into the earth in that way then the Morley-Michelson experiment would not have been able to measure it but if the measurement was taken from a vertical arrangement of the apparatus then they might have found something to measure.

Wouldn't that explain the lack of gravitons?

SOL

I have a followup question...

What is the mechanism for the rotation of plane polarised light travelling through a solution of a chiral (and enantioenriched) substance?

Morbert

himnextdoor wrote: »

Okay but I refer to the Michelson-Morley experiments (and others) which, at the time were trying to settle a two-way dispute regarding the nature of an aether which was presumed to exist and thus, in failing, sounded a death-knoll for the notion of an aether. But I think there could be a third angle to the dispute and a small rearranging of the standard method for the experiment could test for it.

Suppose that matter represents an area depleted of space and trying to equalise spacial pressure space 'pervades' the matter and in doing so provides energy to the system which is radiated off as EMR. This would cause a 'current' of space to bear down in all directions on the mass. The larger the mass, the larger the current.

If the aether were flowing into the earth in that way then the Morley-Michelson experiment would not have been able to measure it but if the measurement was taken from a vertical arrangement of the apparatus then they might have found something to measure.

Wouldn't that explain the lack of gravitons?

Why would mass represent an area of depleted space? Why would pervading space cause EM radiation? Why would it explain the lack of gravitons?

Einstein's field equations have successfully described the coupling of matter and energy to spacetime. And while the Michelson-Morley experiment was one of the first of its kind, many other tests have since been carried out which render an aether superfluous and unnecessary. Furthermore, spacetime has been incorporated into quantum mechanics to produce relativistic quantum field theories like quantum electrodynamics, which have made very precise predictions (An accuracy of one part in a trillion).