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### Author Topic: Speed of light in different media  (Read 7384 times)

#### DoctorBeaver

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##### Speed of light in different media
« on: 23/11/2005 12:28:48 »
The speed of light changes according to the media it is travelling through: the denser the material, the slower the speed.
Now, to my small brain, that doesn't hold water. The reason light appears to slow down is that when it travels through, say, a diamond, the photon is absorbed by a particle in the diamond which vibrates temporarily & then re-emits the photon. That doesn't mean the speed of light is slowed down, merely that its passage from 1 side of the medium to the other takes longer than it would take to travel the same distance in a vacuum.
Analogy. Imagine rolling a ball across a frictionless surface. Let's say it takes the ball 10s to roll 10m. That gives it a speed of 1m/s Now, imagine that there is a person standing halfway. Roll the ball to him. He holds it briefly before rolling it the remaining distance. The ball may well take more than 10s to travel the 10m, so its overall speed is <1m/s. However, whilst in flight the speed of the ball may well exceed 1m/s. The more people there are to catch & pass on the ball (equating to a denser medium), the slower its apparent speed when times from start to finish.
Therefore, it is the speed at which the photon travels between the particles in the medium which should be measured, not its average speed.

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#### Solvay_1927

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##### Re: Speed of light in different media
« Reply #1 on: 23/11/2005 20:36:46 »
Correct.

(Oooh, that sounded authoritative, didn't it!  Of course, I'm only guessing.[:o)])

Think of a white light beam passing through a prism.  The beam is split out into the colours of the rainbow by the time it comes out the other side (because different frequencies of light get delayed by different amounts).  But these coloured beams leaving the prism still move away at the speed of light.  It's not as if they were slowed down by the prism and then continue on at this reduced speed.

According to relativity theory, a photon can never be decelerated to a speed below c (nor accelerated to above c).

So the interesting conundrum is ... how does an electron trap and hold on to a photon (whilst moving up a quantum level), then spontaneously emit it again at speed c (whilst moving down a level again), without contravening relativity (i.e. without decelerating or accelerating the photon)?

Can the photon be thought of as continuing to move at c in some sort of "orbit"/"cloud" around the electron and then flying out of that "orbit" again at speed c (such that it never actually stops moving at speed c)?  It's believed there is a cloud of "virtual" photons surrounding each electron, so does the "real" photon join that cloud?  (Perhaps the real photon "drops by" to have a meal with the virtuals, then rushes off again just before the bill comes?)

Who knows?

We need someone who knows something about quantum physics!

#### turbo-1

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##### Re: Speed of light in different media
« Reply #2 on: 24/11/2005 15:11:48 »
You don't need a quantum theorist, you need an optician.  While you may gain some deeper understanding from quantum physics, classical optics explains this concept quite well.  EM waves are refracted when entering media of different densities.  The amount of refraction is proportional to the difference in density and the angle at which the wave hits the  "surface" of the density gradient.

#### Solvay_1927

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##### Re: Speed of light in different media
« Reply #3 on: 24/11/2005 20:29:04 »
True, Turbo, true - but the question is, what causes the light rays to refract (at the level of the fundamental particles within in the medium) and what happens to the speed of the light rays as they pass through the medium (and how does that fit with relativity theory)?

Classical optics isn't sufficent to explain this (I think?), so you end up in the realm of quantum physics (which can explain it - but unfortunately it explains it in a language I don't understand!)

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##### Re: Speed of light in different media
« Reply #4 on: 24/11/2005 21:55:14 »
Whilst I don't fully understand the following, it is what I recall from reading Brian Greene's Elegant Universe.

When you talk about relativity and the speed of light you should be thinking in terms of 4-dimensional space-time. So maybe, and I'm really probably wrong here, but maybe when we say that the speed of light, c, is constant, we are talking about a 4D velocity, with an x, y and z component, plus a component of speed in the time "direction". Thus if our electron has a "spatial speed" of c in free space, then it has a "temporal speed" of zero - the photon does not "age".

When the electron enters the material then it's spatial speed decreases, but it's 4D speed is still c, as it's temporal speed has increased, such that it is now "aging".

OK, so I am basically suggesting that we can relax the postulate about c being the same in all reference frames by saying that it is only completely true for 4D frames. Or maybe I just mistook what Brian Greene was saying in his book.

#### DoctorBeaver

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##### Re: Speed of light in different media
« Reply #5 on: 25/11/2005 18:05:34 »
My point seems to have been misunderstood. I was pointing out that I've seem it claimed that light travels at different speeds in different mediums. I was questioning that premise by suggesting that the photon still travels between the particles in the medium at c but, due to its interaction with those particles, it takes longer to travel a given distance than were it travelling in a vacuum. So it only appears to travel slower, whereas it is actually still travelling at the same speed it would in a vacuum.

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« Last Edit: 25/11/2005 18:07:12 by DoctorBeaver »

#### Solvay_1927

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##### Re: Speed of light in different media
« Reply #6 on: 25/11/2005 21:04:18 »
Doc - yes, that's what I'm agreeing with - the photons (or rays) pass from one particle to the next at speed c, but they get delayed at each particle (a bit like when I go cruising around the local red light district, I keep a constant speed but slow down as I pass each ... er ... sorry, too much information, strike that[:I]).

So the light takes 1 microsecond (say) to pass through the medium, but that's actually made up of a total of 0.1 microsecs of travelling at c in the empty spaces between the particles and a total of 0.9 microsecs of "hanging out" with the particles along the way.

(But then - and this is going off at a tangent, admittedly - that leads to the dilemma of how can the photon be slowed/stopped in the vicinity of each particle without violating relativity?)

Or have I still not got what you're saying?

Adam - welcome to the forum.  I'm not an expert (far from it), but I'm pretty sure that relativity does require the speed of light (in a vacuum) to be invariant in 3D space in all inertial frames.  That's actually one of the fundamental postulates that the whole theory is based on.
What you're describing may apply to electrons and things that travel at less than c, but not to photons of light.  (But I may be wrong.)

Any real physicists out there who can help?

#### Dr. Praetoria

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##### Re: Speed of light in different media
« Reply #7 on: 25/11/2005 21:10:16 »
Think of this light wave as a "line of people all holding hands" (in a straight line) and moving across a smooth grass area.  Now part of this line meets some rough area to walk on--they will be slowed down (velocity reduction) causing the line to be bent (refracted).
Doc

#### turbo-1

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##### Re: Speed of light in different media
« Reply #8 on: 25/11/2005 21:45:40 »
quote:
Originally posted by Solvay_1927

True, Turbo, true - but the question is, what causes the light rays to refract (at the level of the fundamental particles within in the medium) and what happens to the speed of the light rays as they pass through the medium (and how does that fit with relativity theory)?

Classical optics isn't sufficent to explain this (I think?), so you end up in the realm of quantum physics (which can explain it - but unfortunately it explains it in a language I don't understand!)

We have to consider WHAT in "empty" space is being "bent" (densified) by matter, giving rise to the effects refraction that we call gravitational lensing.  What suffuses all of space? The ZPE fields of the quantum vacuum.  EM traverses this "empty" space to get from luminous bodies in the universe to our eyes and sensors.

#### Solvay_1927

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##### Re: Speed of light in different media
« Reply #9 on: 26/11/2005 01:39:25 »
Doc (the other one this time - Praetoria, not Beaver) - good analogy, it helps explain light speed reducing when thinking of light as a wave.  But what about when a single photon is sent through a medium and gets slowed down?
(But then again, I suppose that's the problem with quantum physics - light isn't actually made of classical particles nor classical waves - it's something in-between/both at the same time - so it's impossible to fully describe it using classical images.)

Turbo (I forgot to say welcome to the forum to you too) - I fear your understanding may be somewhat more advanced than mine and I may be out of my depth here!  Can you expand / give any web links please?  (Am I right in thinking that you're saying the following: the photons are travelling at c throughout their journey, but that the medium they pass through is warping the space they pass through, so they appear to us to take longer to pass through the medium?  If so, that sounds like a good explanation too.)

#### DoctorBeaver

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##### Re: Speed of light in different media
« Reply #10 on: 26/11/2005 14:58:34 »
When a photon interacts with a particle, is it the same photon that is re-emitted? If the original photon is absorbed & another emitted in its place, then c will not be violated.
I suggest this as I believe photons can be emitted by certain particles being excited. I can't imagine those particles have lots of little photons inside them queueing up to be emitted, so how are they formed? & doesn't this infer that photons interacting with a particle are somehow absorbed and transformed to become a different kind of particle, or constituent parts of other particles?

#### DoctorBeaver

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##### Re: Speed of light in different media
« Reply #11 on: 26/11/2005 15:02:10 »
Solvay - I was referring to Turbo's mentioning refraction as being the misunderstanding. I'm fully aware of what refraction is but I don't see that it bears any relevance to my initial point. As far as I'm aware, refraction changes the frequency but not the speed of photons
« Last Edit: 26/11/2005 15:02:54 by DoctorBeaver »

#### Soul Surfer

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##### Re: Speed of light in different media
« Reply #12 on: 26/11/2005 18:40:25 »
I think that you are all coming slowly to the right conclusion. The problem is one of stretched analogies as many of these physical probems turn out to be.  You should only use an analogy in the area where it is relevant.

The interaction of electromagnetic radiation with materials is complex (in more senses thamn one :-)

The important thing to consider is the wavelength of the radiation involved.

If you are considering normal solid objects like lumps of glass or metal and longer wavelengths like light and radio, the interaction is mostly with the whole material because the wavelength of the electromagnetic radiation spans across quite a lot of atoms simultaneously.  When you use shorter wavelengths like X and gamma rays the fields can "see" the individual atoms and go in between them.  (that's why Xrays go through you)

An interesting aside is that with even longer wavelengths like uhf and microwaves you can create artificial materials by using periodic structures like atoms arranged in a solid.  An example of this is the stanard Yagi type of TV aerial.  The real aerial is the little loop near the back  The bars right at the back are a mirror reflecting the signal back on to the  loop and the bars along the front are an artificial dielectric acting like a lens to focus the signal on to the loop.

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#### Solvay_1927

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##### Re: Speed of light in different media
« Reply #13 on: 28/11/2005 14:48:55 »
quote:
When a photon interacts with a particle, is it the same photon that is re-emitted? If the original photon is absorbed & another emitted in its place, then c will not be violated.
I suggest this as I believe photons can be emitted by certain particles being excited. I can't imagine those particles have lots of little photons inside them queueing up to be emitted, so how are they formed? & doesn't this infer that photons interacting with a particle are somehow absorbed and transformed to become a different kind of particle, or constituent parts of other particles?

Eth - you're right, good point - it doesn't have to be the same photon emitted, it could be a different one. But then what happens to it when it's absorbed, and where does the new photon emitted come from? The electrons could have "photons inside them queueing up to be emitted" - in the form of a surrounding cloud of virtual photons popping into and out of existence, perhaps - but we don't know.  (And, even if this cloud was real, we can't understand the mechanism that causes photons to get trapped/emitted by it.)

I still think we have a conundrum as far as relativity goes, though.  Even if the emitted photon is different from the incoming one, the incoming one still has to be decelerated (stop dead) and the new one accelerated (from standing) - but relativity suggests nothing travelling at c can be decelerated nor accelerated by any force.  (Well, that's my understanding anyway.  So it may be wrong.)

As an aside ... One thing I've always puzzled over is that, if a material has any temperature at all (i.e. it's greater than -273C), then surely it must be emitting photons all the time (because it's giving off EM radiation).  So as long as a substance is above absolute zero, the atoms in it are "excited" and must be emitting (and absorbing) photons all the time.  (Or is my thinking wrong on this?)
Which makes me wonder how accurate experiments on single photons can really be. (E.G. in the double-slit experiment, how do you know the individual photon you're sending through the apparatus is the only one there?  The screen and the detectors must be emitting photons too, mustn't they?)

Paul.

#### Soul Surfer

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##### Re: Speed of light in different media
« Reply #14 on: 29/11/2005 10:38:09 »
Yes they are but at a very different frequency to the ones that you are observing.  To emit light. objects have to be at thousands of degrees K while at room temperature objects emit mostly longish infra red frequencies

hot objects also emit longer wavelength radiation but there is a sharpish cut off for shorter wasvelength (higher energy) photons.

Re the absorbtion and emission of photons, when a photon is absorbed it is effectively destroyed and its energy used for someting else like incresing the angular momentum of the electron in its orbit around the nucleus (raising the energy level in the atom)  when it iemitted the oscillating electromagnertic field associated with the electron moving to a different quantised orbit creats a new photon.  The idea of having a whole load of photons sitting inside an electron is not a good one for visualising the process.  it is better to think of an electron that is in a stable orbit is continually emitting and reabsorbing a photon that would cause its energy level to change

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« Last Edit: 29/11/2005 10:41:15 by Soul Surfer »

#### DoctorBeaver

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##### Re: Speed of light in different media
« Reply #15 on: 29/11/2005 19:16:12 »
quote:
Re the absorbtion and emission of photons, when a photon is absorbed it is effectively destroyed and its energy used for someting else like incresing the angular momentum of the electron in its orbit around the nucleus (raising the energy level in the atom) when it iemitted the oscillating electromagnertic field associated with the electron moving to a different quantised orbit creats a new photon. The idea of having a whole load of photons sitting inside an electron is not a good one for visualising the process. it is better to think of an electron that is in a stable orbit is continually emitting and reabsorbing a photon that would cause its energy level to change

That's what I thought. So there is no deceleration involved. But what about the creation/emission side of it? The photon must be travelling at c from the instant of its creation. But surely the infinite energy thing cannot only apply to acceleration. Wouldn't the creation of something travelling at c also require infinite energy? It seems to me that it would be a bit of a nonsense if that were not the case.

#### turbo-1

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##### Re: Speed of light in different media
« Reply #16 on: 02/12/2005 15:54:04 »
quote:
Originally posted by DoctorBeaver

Solvay - I was referring to Turbo's mentioning refraction as being the misunderstanding. I'm fully aware of what refraction is but I don't see that it bears any relevance to my initial point. As far as I'm aware, refraction changes the frequency but not the speed of photons

Refraction is a result of changes in the speed of light as it transitions through media of varying densities.  Ideally, refraction does not change the frequency of the light involved.

In classical optics, light propagates as EM waves, and the waves are refracted (bent) when they encounter a transmissive medium of higher or lower density than they one they are currently propagating through.  The amount of refraction depends on the density difference and the steepness of the angle of incidence at which the wavefront impinges on the density gradient.  Light propagates more slowly through denser media (higher refractive index) than through rarified media.

#### DoctorBeaver

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##### Re: Speed of light in different media
« Reply #17 on: 02/12/2005 17:11:12 »
quote:
Light propagates more slowly through denser media (higher refractive index) than through rarified media.

This is precisely my point. Does the light actually travel slower or just take longer to pass through?

#### neilep

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##### Re: Speed of light in different media
« Reply #18 on: 02/12/2005 17:50:27 »
I saw this program about the construction of the biggest particle accelerator ever and that they will be accelerating particles to vey close to the speed of light travelling in opposite directions and then slamming them into each other..Oh fun !..these things scare the living daylights out of me as I expect them to create another big bang which can really ruin your day !..anyway...won't the combined speed of these colliding particles be faster than the speed of light ?...I figure you'll probably say No eh ?

sorry to go off topic a little...but I'm also off...more than a little !!

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« Last Edit: 02/12/2005 17:57:42 by neilep »

#### DoctorBeaver

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##### Re: Speed of light in different media
« Reply #19 on: 02/12/2005 18:08:18 »
quote:
won't the combined speed of these colliding particles be faster than the speed of light ?...I figure you'll probably say No eh ?

No

#### DoctorBeaver

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##### Re: Speed of light in different media
« Reply #20 on: 02/12/2005 18:12:20 »
Neil - you're forgetting to take the time dilation aspect of relativity into account. Even if 2 particles travelling at c collide head-on, their closing velocity will still only be c.
But think about this - at c the entire life of the universe passes in zero time. That means that the collision of those particles will last forever! [:o)]

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##### Re: Speed of light in different media
« Reply #20 on: 02/12/2005 18:12:20 »