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Author Topic: How does light speed up when it exits a denser material?  (Read 27096 times)

Offline thedoc

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How does light speed up when it exits a denser material?
« Last Edit: 16/06/2015 17:07:07 by Georgia »


 

Offline syhprum

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Re: How does light speed up when it exits a denser material?
« Reply #1 on: 13/03/2014 20:58:50 »
Photons never move at other than light speed the apparent slowing down is due to the tortuous path they take being absorbed and re-emitted by atoms en route.
When they leave the materiel this effect ceases and their normal speed becomes apparent.
I know that this a simplified version but the gist is correct.
« Last Edit: 14/03/2014 21:23:55 by syhprum »
 

Offline annie123

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Re: How does light speed up when it exits a denser material?
« Reply #2 on: 14/03/2014 19:43:51 »
Re the speed of light in general - I have asked elsewhere on this forum about the scientist who has slowed down light and even stopped it at one point, and why this is not being celebrated/discussed/questioned/debated/explored more. Is this scientist's discovery (Lene Haus) not that important after all? I thought that showing Einstein was wrong would be a big deal.
 

Offline evan_au

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Re: How does light speed up when it exits a denser material?
« Reply #3 on: 15/03/2014 01:44:32 »
Quote
I thought that showing Einstein was wrong would be a big deal.
Einstein's theories have been remarkably resilient at large scales. (Relativity does not address the quantum scale, although his work on the photoelectric effect does.)

We routinely slow down light slightly by shining it through a transparent medium like glass - this is how lenses work.

Slowing light to a walking pace or slower does not violate Einstein's theory of relativity, which takes the speed of light in a vacuum as a speed limit for matter.

These demonstrations of slowing light uses light of a very specific wavelength, and a medium which is carefully primed to put it in the right quantum state.
It is definitely not a vacuum...
 

Offline thedoc

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Hear the answer to this question on our show
« Reply #4 on: 16/06/2015 16:00:39 »
We discussed this question on our  show
We put this question to Cambridge University physicist Zephyr Penoyre...
Zephyr - When it goes into the glass, it’s moving slower. But also, because the frequency has to be the same, the same waves have to be coming into the glass at the same rate as they're coming out, otherwise you’ve lost waves somewhere along the way…
Chris - And it’s changed colour.
Zephyr - And that’s because wavelength has changed. So, the energy of the light is to do with the wavelength of the light. So, as it slows down, the energy gets higher - because the wavelength has shrunk - so the total amount of energy passing through the glass is exactly the same as it was going through the air, it's just moving at a slightly different speed.
Click to visit the show page for the podcast in which this question is answered. Alternatively, listen to the answer now or [download as MP3]
« Last Edit: 01/01/1970 01:00:00 by _system »
 

Offline PmbPhy

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Re: How does light speed up when it exits a denser material?
« Reply #5 on: 16/06/2015 18:03:03 »
Quote from: evan_au
Relativity does not address the quantum scale, although his work on the photoelectric effect does.
What do you mean? There is a relativistic theory of quantum mechanics.
 

Offline Colin2B

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Re: How does light speed up when it exits a denser material?
« Reply #6 on: 16/06/2015 19:07:19 »
Re the speed of light in general - I have asked elsewhere on this forum about the scientist who has slowed down light and even stopped it at one point, and why this is not being celebrated/discussed/questioned/debated/explored more. Is this scientist's discovery (Lene Haus) not that important after all? I thought that showing Einstein was wrong would be a big deal.
The work of Lene Hau is being celebrated but it would be a mistake to conclude that it shows Einstein to be wrong. It is also a mistake the think that Einstein said light speed is constant in all circumstances, the constancy of light speed as described by Einstein is for a vaccum. Dr Hau's work was on light in dense, indeed very cold, media eg sodium, for which she has received significant and well deserved accolades.
 

Offline evan_au

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Re: How does light speed up when it exits a denser material?
« Reply #7 on: 17/06/2015 11:45:03 »
Quote from: Zephyr
So, as (light) slows down, the energy gets higher
This seems to be talking about the energy density of a beam of light, eg measured  Joules/m3 or similar

However, when you are talking about an individual photon, its energy remains the same in vacuum, air or glass.

Unlike objects having a rest mass, light does not gain or lose energy when its speed changes, so it doesn't need to gain energy when it exits the glass.
 

Offline evan_au

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Re: How does light speed up when it exits a denser material?
« Reply #8 on: 17/06/2015 11:55:29 »
Quote from: PmbPhy
There is a relativistic theory of quantum mechanics.

Yes, quantum mechanics has been adapted to consider relativistic effects such as c being a speed limit, time dilation and gravitational red shifting, etc. All these can be tested on Earth, for example by accelerating subatomic particles to nearly the speed of light in a particle accelerator, or by flying atomic clocks around the Earth in a GPS satellite.

However, I have not seen a well-tested quantized theory of relativity. To test this would require close study of the behaviour of subatomic particles close to the event horizon of a black hole. If scientists attempted experiments on a reasonable-sized black hole here on Earth, I fear that they would not live long enough to publish the paper.
 

Offline lightarrow

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Re: How does light speed up when it exits a denser material?
« Reply #9 on: 17/06/2015 13:32:51 »
Quote
How does light speed up when it exits a denser material?
It doesn't. Light which exits the material it's not the same light which enters and which is inside: the em field which exits is generated by the last charges inside the material.

--
lightarrow
 

Offline jeffreyH

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Re: How does light speed up when it exits a denser material?
« Reply #10 on: 18/06/2015 00:29:30 »
At the event horizon of a black hole the gravitational field exhibits a critical density like a 'perfect' medium. Perfect in respect of its ability to stop light. The photon's energy is effectively canceled exactly at the horizon. At this point the electromagnetic and gravitational fields are at equilibrium. Not in the sense of a stationary point but in the balance of energies. This does not help unless observational data can be collected from the immediate environment surrounding a black hole. A collision with the horizon of Sag A* is the best chance of obtaining this data.
 

Offline Bill S

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Re: How does light speed up when it exits a denser material?
« Reply #11 on: 18/06/2015 15:13:40 »
Quote from: Lightarrow
Light which exits the material it's not the same light which enters and which is inside: the em field which exits is generated by the last charges inside the material.

Explanations often say, or imply, that it is not a single photon that travels through the medium (e.g. glass); it is a succession of new photons, created at each new emission from atoms in the glass.  If this were what actually happened, then the absorption spectrum would be discrete because atoms have only discrete energy states. Yet, in glass for example, we see almost the whole visible spectrum being transmitted with no discrete disruption in the measured frequencies.

The reason for this is that a solid is composed of a network of ions and electrons fixed in a "lattice".  They have what is known as "collective vibrational modes", or phonons. These are quanta of lattice vibrations, and it is these vibrational modes that can absorb a photon. 

If a photon has an energy beyond the phonon spectrum, the solid cannot sustain this vibration, because the phonon mode is not available; so the lattice does not absorb this photon and it is re-emitted but with a very slight delay. 
 

Offline lightarrow

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Re: How does light speed up when it exits a denser material?
« Reply #12 on: 20/06/2015 12:35:47 »
Quote from: Lightarrow
Light which exits the material it's not the same light which enters and which is inside: the em field which exits is generated by the last charges inside the material.

Explanations often say, or imply, that it is not a single photon that travels through the medium (e.g. glass); it is a succession of new photons, created at each new emission from atoms in the glass.  If this were what actually happened, then the absorption spectrum would be discrete because atoms have only discrete energy states. Yet, in glass for example, we see almost the whole visible spectrum being transmitted with no discrete disruption in the measured frequencies.

The reason for this is that a solid is composed of a network of ions and electrons fixed in a "lattice".  They have what is known as "collective vibrational modes", or phonons. These are quanta of lattice vibrations, and it is these vibrational modes that can absorb a photon. 

If a photon has an energy beyond the phonon spectrum, the solid cannot sustain this vibration, because the phonon mode is not available; so the lattice does not absorb this photon and it is re-emitted but with a very slight delay. 

Yes, this is also what I quoted from another forum in this one some years ago. Infact I haven't written that a photon is absorbed by an atom's material then ri-emitted and so on.

Nontheless a photon entering the material is not the same photon exiting from it: the em field, entering the material, let's say glass, excites it and the glass generates in turn another em field, which interacts with the first and with glass itself. Photons are constantly created and destroyed in the resulting field so their individuality is lost at the end of the path.

--
lightarrow
 

Offline Bill S

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Re: How does light speed up when it exits a denser material?
« Reply #13 on: 20/06/2015 22:36:02 »
Quote from: Lightarrow
Infact I haven't written that a photon is absorbed by an atom's material then ri-emitted and so on.

You are right, of course.  These thing always seem to get more complicated, the closer you look at them.
 

Offline RTCPhysics

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Re: How does light speed up when it exits a denser material?
« Reply #14 on: 25/06/2015 10:01:09 »
Reading through the views expressed, there seemed to be three alternative explanations put forward.

1.   The ‘bus stop’ theory, where like a bus stopping at every bus stop, a light photon stops at every electron it meets on its route through the translucent material.
2.   The ‘spring’ theory, where the photon acts like a spring which becomes more tightly coiled on its passage through the translucent material.
3.   The ‘water surfer’ theory, where like a surfer skimming the wave tops of water, the photon is swept through the translucent material carried on the top of the internal wave motion of the material’s ‘electron sea’.

The arguments raised against each particular theory were along the following lines:

The ‘bus stop’ theory needs to explain how all visible light photons are absorbed and emitted by electrons on their passage through the translucent material. Particular elements such as silicon, have a specific absorption and emission spectra, so some visible light frequencies would not be affected and pass through at the speed of light without hindrance.
 
The ‘coiled spring theory’, raised the question of where the frequency of a photon of visible light changes to a higher value. Is it upon contact with one or multiple electrons on its passage through the translucent material? And interestingly, if an infra-red photon passed through the translucent material would it be visible on its passage as ‘red light’?

The ‘water surfer’ theory makes the assumption that there is a ‘standing wave’ within the ‘electron sea’, which carries the photon along without absorption and emission. (This is articulated much more cogently as the collective vibrational modes (phonons) of the ions and electrons located in their lattice structure.) The speed of passage of the photon through the translucent material riding this vibrational lattice must be presumably less than the speed of light, but the photon itself is unaffected and emerges at the speed of light again.

All three theories are plausible, but it does seem to me that our understanding of the photon and its interaction with the electron is at best sketchy. If we understood the commonality of the energy structures of the photon and the electron, we would know how the photon is absorbed and released by the electron, (bus stop theory), receives and releases energy, (coiled spring theory) and hitches a lift without energy being exchanged with the electron lattice (water surfer theory).


 

Offline chiralSPO

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Re: How does light speed up when it exits a denser material?
« Reply #15 on: 25/06/2015 14:25:30 »
And interestingly, if an infra-red photon passed through the translucent material would it be visible on its passage as ‘red light’?

No, the frequency determines the "color," and that does not change from medium to medium. The wavelength must change, but ultimately it is the frequency (energy) that registers as the color in our eye.

If you consider the eye as the detector, then it doesn't matter what material the light passes through, ultimately the refractive index of the eye where the light is absorbed determines the "observed wavelength", but we can design an experiment to probe the color of light in other materials:

Say we have two solutions that each contain the same concentration of a dye that absorbs at a very specific frequency, but the two solutions also contain different amounts of some other dissolved species that changes the refractive index substantially (many additives could allow us to tune the refractive index by 10-20%, without changing the behavior of the dye). If the spectrum of light coming out of the solution is independent of the refractive index, that proves that the frequency determines color, not the wavelength (and if the absorbed light shifts with refractive index, that proves the wavelength is the important factor)
 

Offline RTCPhysics

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Re: How does light speed up when it exits a denser material?
« Reply #16 on: 24/07/2015 13:30:18 »
Quote
No, the frequency determines the "color," and that does not change from medium to medium. The wavelength must change, but ultimately it is the frequency (energy) that registers as the color in our eye.

If you consider the eye as the detector, then it doesn't matter what material the light passes through, ultimately the refractive index of the eye where the light is absorbed determines the "observed wavelength", but we can design an experiment to probe the color of light in other materials:

Say we have two solutions that each contain the same concentration of a dye that absorbs at a very specific frequency, but the two solutions also contain different amounts of some other dissolved species that changes the refractive index substantially (many additives could allow us to tune the refractive index by 10-20%, without changing the behavior of the dye). If the spectrum of light coming out of the solution is independent of the refractive index, that proves that the frequency determines color, not the wavelength (and if the absorbed light shifts with refractive index, that proves the wavelength is the important factor)

I disagree. The explanation that the reduction of the velocity of visible light in a translucent material is effected by a reduction in wavelength but not frequency, implies that the sinusoidal wave model of a single photon is incorrect. With the sinusoidal model, the wavelength and frequency are related, one being the inverse of the other.

The only way I can see to explain the reduction of light speed upon entering a translucent material from say, air, is if the amplitude of the sinusoidal wave is increased at the boundary between the media, but its frequency and wavelength remain unchanged. The higher amplitude increases the three dimensional distance travelled by the photon's new sinusoidal wave form and hence its velocity through the medium. On emerging from the translucent medium back into air, the amplitude reverts to its previous level, giving the photon the energy it needs to return to its previous velocity.

Reading the results of experimental tests upon the functioning of the eye, their results all refer to the wavelength of the visible light as being the important factor in the colours that the brain observes, not frequency.

Absorption and refraction are two different phenomenon, not to be confused. Refraction or bending of light occurs at the surface boundary between the two media. Absorption of a particular frequency of light by electrons can happen anywhere within the receiving medium. In the experiment you described, adding the dye to the change of refractive index would only shift the whole light spectrum to one side. The absorption frequency of the translucent solution would be unchanged within this spectrum, proving nothing. Except that the added dye does not absorb light of the particular frequency used in the experiment. I'm assuming that the absorption line did not move!
 

Offline jeffreyH

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Re: How does light speed up when it exits a denser material?
« Reply #17 on: 24/07/2015 13:51:29 »
I have never read anywhere that wave amplitude is a factor. Do you have any references for that?
 

Offline chiralSPO

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Re: How does light speed up when it exits a denser material?
« Reply #18 on: 24/07/2015 15:29:00 »

I disagree. The explanation that the reduction of the velocity of visible light in a translucent material is effected by a reduction in wavelength but not frequency, implies that the sinusoidal wave model of a single photon is incorrect. With the sinusoidal model, the wavelength and frequency are related, one being the inverse of the other.

They are not quite inverses, they are inverse AND related by the velocity: wavelength = speed / frequency (wavelength * frequency = speed ; meters per cycle times cycles per second equals meters per second) Therefore, to maintain a fixed frequency when light slows down, the wavelength must decrease. This can be (and has been) experimentally shown by performing interferometry (which depends on the wavelength, not the frequency) in media other than vacuum, and comparing against the energy of the photon (which is a function of frequency, not wavelength).

The only way I can see to explain the reduction of light speed upon entering a translucent material from say, air, is if the amplitude of the sinusoidal wave is increased at the boundary between the media, but its frequency and wavelength remain unchanged. The higher amplitude increases the three dimensional distance travelled by the photon's new sinusoidal wave form and hence its velocity through the medium. On emerging from the translucent medium back into air, the amplitude reverts to its previous level, giving the photon the energy it needs to return to its previous velocity.

I don't think amplitude has anything to do with this. The sinusoidal wave is not the "path" that the photon takes. It is a representation of the electromagnetic oscillation of the photon. More photons = greater amplitude (if they're in phase).

Reading the results of experimental tests upon the functioning of the eye, their results all refer to the wavelength of the visible light as being the important factor in the colours that the brain observes, not frequency.

Almost everybody talks about the wavelength of light rather than frequency, but it is implied that they mean the wavelength in a vacuum. The wavelength changes based on the refractive index of the medium the light is in (see above), but the frequency (energy) remains constant. The eye detects light by virtue of rhodopsin, which is the adduct of an enzyme (opsin) and a small molecule called retinal. The light isomerizes retinal, causing a conformational change in the rhodopsin, which sends a signal to the nerve, etc. etc. There are a few different variants of opsin (called photopsins) that can modulate the energy required to isomerize the retinal (this is how we see different colors). The key here is that the differentiation of different "wavelengths" of light is due to little protein devices that respond very selectively to certain energies (frequencies) of light.

(https://en.wikipedia.org/wiki/Rhodopsin)
(https://en.wikipedia.org/wiki/Photopsin)

Absorption and refraction are two different phenomenon, not to be confused. Refraction or bending of light occurs at the surface boundary between the two media. Absorption of a particular frequency of light by electrons can happen anywhere within the receiving medium. In the experiment you described, adding the dye to the change of refractive index would only shift the whole light spectrum to one side. The absorption frequency of the translucent solution would be unchanged within this spectrum, proving nothing. Except that the added dye does not absorb light of the particular frequency used in the experiment. I'm assuming that the absorption line did not move!

This is exactly my point! If the absorption line did not move, and the whole spectrum shifts to one side, the light that gets absorbed depends on how much of a shift there was!!!  ;D
 

Offline lightarrow

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Re: How does light speed up when it exits a denser material?
« Reply #19 on: 25/07/2015 09:22:04 »
I disagree. The explanation that the reduction of the velocity of visible light in a translucent
"Translucent"? it's not the correct term.
Quote
material is effected by a reduction in wavelength but not frequency, implies that the sinusoidal wave model of a single photon is incorrect
"sinusoidal wave model of a single photon"? It doesn't exist. Maybe you intended something else.
Quote
. With the sinusoidal model, the wavelength and frequency are related, one being the inverse of the other.
No, as others already wrote, in an electromagnetic wave, not only wavelength λ and frequency f are related, but radiation velocity (precisely phase velocity vp) too:

λ = vp/f

frequency f doesn't change inside the material, so if vp decreases, also does λ.


Quote
The only way I can see to explain the reduction of light speed upon entering a translucent material from say, air, is ...
... your personal and wrong theory.

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lightarrow
« Last Edit: 25/07/2015 09:24:43 by lightarrow »
 

Offline evan_au

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Re: How does light speed up when it exits a denser material?
« Reply #20 on: 26/07/2015 08:15:03 »
Quote from: ChiralSPO
The wavelength changes based on the refractive index of the medium the light is in (see above), but the frequency (energy) remains constant
I heard a very practical example of the fact that the wavelength of light changes when it enters a material of higher refractive index:
  • Obtaining the light spectrum of stars and planets is a powerful tool for astronomers - it allows them to determine their chemical composition without actually traveling there to take a sample.
  • One of the common ways to break light up into a spectrum is to use a diffraction grating - very fine lines etched on a flat surface, which breaks up the light into a rainbow of its constituent colors.
  • Astronomers studying the infra-red need to put their instruments above the atmosphere, ideally in a satellite, but volume is at a premium in a satellite.
  • Some instrument makers recently "reversed" the usual diffraction grating design - instead of shining light onto the diffraction grating in a vacuum, they shone the light through the back of a silicon wafer onto the diffraction grating, so the light was passing through silicon, rather than through a vacuum when it hit the grating.
  • The refractive index of silicon is about 3-5 times higher than a vacuum in the infra-red region, so the wavelength of light is correspondingly shorter.
This allowed them to separate the wavelengths as effectively as an instrument about 4 times longer (and wider & high), so they reduced the volume of the instrument by a factor of about 60, which is a significant saving in a spacecraft! 

Quote from: Lightarrow
a photon entering the material is not the same photon exiting from it: the em field, entering the material, let's say glass, excites it and the glass generates in turn another em field, which interacts with the first and with glass itself. Photons are constantly created and destroyed in the resulting field so their individuality is lost at the end of the path.
Maybe I'm just nitpicking here, but...
I would call a photon an em field which propagates through space, as per Maxwell's equations.

To me, it doesn't matter whether the em field at point X is the same em field that exists 1 meter (or 1 micrometer) away. What matters is that it is the same quantum of energy that is propagating (as per Einstein's explanation of the photoelectric effect). To me, the photon that enters a glass block is the same photon that exits a glass block (assuming it isn't absorbed in-between).
 

Offline Thebox

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Re: How does light speed up when it exits a denser material?
« Reply #21 on: 26/07/2015 08:29:00 »
Presuming light has a speed and it is not something in space that is being energized at a speed such as CBMR.
 

Offline evan_au

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Re: How does light speed up when it exits a denser material?
« Reply #22 on: 26/07/2015 12:13:09 »
Quote from: Thebox
Presuming light has a speed
Light definitely has a speed, as measured by Ole Rømer in 1676, Fizeau & Focault around 1848 and many more modern measurements.
In Physics, we call this speed "c", and it is about 300 thousand km/s in a vacuum.

See: https://en.wikipedia.org/wiki/Speed_of_light#Measurement

Quote
Presuming light ...is not something in space that is being energized at a speed such as CBMR.
Visible light happens in space because of things like stars, bouncing off things like planets, Moons and clouds of dust. It does not rely on the CMBR to power it (the CMBR is far too weak to have much effect on visible light).
« Last Edit: 26/07/2015 14:46:01 by chiralSPO »
 

Offline lightarrow

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Re: How does light speed up when it exits a denser material?
« Reply #23 on: 26/07/2015 20:23:53 »
Quote from: Lightarrow
a photon entering the material is not the same photon exiting from it: the em field, entering the material, let's say glass, excites it and the glass generates in turn another em field, which interacts with the first and with glass itself. Photons are constantly created and destroyed in the resulting field so their individuality is lost at the end of the path.
Maybe I'm just nitpicking here, but...
I would call a photon an em field which propagates through space, as per Maxwell's equations.
Aha! 90 yars of quantum mechanics thrown away! 😊
Sorry but a photon is something much more complicated than that!
Quote
To me, it doesn't matter whether the em field at point X is the same em field that exists 1 meter (or 1 micrometer) away. What matters is that it is the same quantum of energy that is propagating (as per Einstein's explanation of the photoelectric effect).
Then an electron or a photon is the same, for you 😊
Quote
To me, the photon that enters a glass block is the same photon that exits a glass block (assuming it isn't absorbed in-between).
Prove it, if you can.

--
lightarrow
 

Offline evan_au

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Re: How does light speed up when it exits a denser material?
« Reply #24 on: 26/07/2015 22:40:47 »
Quote
Then an electron or a photon is the same, for you
No, an electron does not obey Maxwell's equations (in particular, it cannot travel at c); a photon does.

Quote
Prove it, if you can.
I don't think it is a matter of proof. I think it comes down to how it is defined.

I don't know what would be an authoritative definition, but in this one: "a photon is a discrete bundle (or quantum) of electromagnetic (or light) energy", ..the focus is on the photon as a quantum of energy, rather than on the photon as a localized disturbance in the em field.

More authoritative definitions & interpretations are welcome!
« Last Edit: 27/07/2015 10:53:00 by evan_au »
 

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Re: How does light speed up when it exits a denser material?
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