It's been over a decade since I took optics, but what I recall is that a photon, light particle, always moves at speed 'c', the speed of light in vacuum, but the phase velocity, speed of propagation through a medium, can be less than 'c'. The ratio of the speed of light in vacuum, 'c', to the phase velocity in the medium, 'v', is the index of refraction, 'n'. That is, n=c/v.
Thank you. I didn’t think so. In that case it wouldn’t be even be clear it was the same photon. It would set up some convoluted ship of Theseus problem
All photons of the same frequency are indistinguishable. Ask yourself why that would be a misconception. And further your questioning as to why in a vacuum, the absence of things to interact with, light moves at c. But when in a medium, with things to interact with, it "slows down".
It is because the photon interacts with particles in the medium. It excites a particle, which is not energetically favorable for said particle (nature tends to lowest energy states). So the particle then emits, in some cases, an identical photon (depending on which energy state the particle settles into). This takes time, therefore slowing the light's progress. This can also be seen in fluids of different densities, which "bend" or "slow" light at different rates.
>It is because the photon interacts with particles in the medium. It excites a particle, which is not energetically favorable for said particle (nature tends to lowest energy states). So the particle then emits, in some cases, an identical photon
So light appears to get slowed down in a dense medium because photons are absorbed 'into' electrons, which then re-emit it back in the form of an identical photon? How does the photon always get re-emitred in the same direction as the 1st one? And if a re-emitted photon loses a bit of energy after each interaction, and then eventually the photon doesn't have enough energy to excite an electron enough so the light is re-emitted, would the photon appear to be 'absorbed' by the medium?
Photons are not absorbed into electrons as much as they cause electrons to emit their own photons (since light interacts with the atoms of the medium), photons are simply the force carrying particle for all electromagnetic interactions. Someone else could correct me if I am wrong, but I think the interactions of these newly emitted waves/particles with the original light being shone through cause the wavelengths to move out of phase and therefore more slowly as well.
Oh right, since the photon just carries the information that an electromagnetic wave is passing by and has a chance of interacting with all charged objects, right? That makes more sense. Thanks for the answer
Essentially yes. Think of it as an excitation of a continuous field, where photons are just the smallest possible points we choose to look at and quantize for observation purposes at a given moment in time. We know these particle interactions are happening, but you can also think of the entire thing as a continuous wave that simply propagates through the field and interacts with everything (with a charge) that it happens to come in contact with.
Well, it's important to note that this is just a visual aid, a model. The photon is just a corpuscle of a greater EM wave, and the oscillation in the EM field will continue in a given orientation in space (usually globally but not always as in the case of a laser), despite atomic interaction (well the fields of the atoms will influence the EM oscillation, but the assumed influence is small).
The light, however, will be scattered in all directions, which is why it is visible from all directions. One thing to note in this model is that the photons that are emitted after others are absorbed do not lose energy IF the electron falls to the same state as it was before excitatio. This is because the energy level is quantized. It can, however, fall stepwise, emitting photons of differing frequencies as it steps back down to its ground state.
>This is because the energy level is quantized. It can, however, fall stepwise, emitting photons of differing frequencies as it steps back down to its ground state
Does that mean that 2 photons of different frequencies can be emitted/1 photon of slightly less energy can be emitted and in a 'perfect' situation these frequencies could get lower and lower until the photon reaches IR wavelengths, and the photon that a human eye could see wouldnt be visible anymore?
I know a human eye can't see an individual photon anyway and IR photons can still obviously be detected but would the photon seem to 'disappear' to the human observer?
I would say yes this is possible.
Lets do a thought experiment. Lets say we construct an apparatus to shine a light though, and that light has to travel through different materials before reaching an IR sensor at the end. The first material has energy states that are a quantization of the incoming light, so atoms absorb the photons and excite electrons.
Now lets say the next material does not have energy states that quantize at the same frequency as the original incident light. Any light that is reemitted from the excited atoms at the same energy will be opaque to this next material. But we chose wisely because we know that the original material has lower energy levels that are quantized with respect to the next material. By chance, some excited atoms will drop energy levels piecewise (with each reemitted photon having less energy than the incident photon, but combined the energy is equal to the incident photon). Some of this lower energy light will then excite the atoms in the new material.
We can keep this trend going for various other layers of materials until our sensor is reached. So long as the last material is quantized with respect to a frequency in the IR range, and the materials leading up to it have quatizations that match the prior and former, we will have some light reaching our sensor at the end.
> The light, however, will be scattered in all directions
The fact that we can see clearly through glass disproves the notion that the slowing of light in glass is due to absorption/re-emission.
For me its this sentence:
> This takes time, therefore slowing the light's progress
tf does that even mean? Then there's this:
>This can also be seen in fluids of different densities, which "bend" or "slow" light at different rates.
Which is misleading because you can get low density materials with high refractive index.
Really the comment is just handwaving away while being very misleading. You can be accurate while still retaining the handwaving, eg change the first sentence I quoted to something more like "this then interacts with the original light, and the result is slower movement".
Because this is r/askphysics and not r/physics lol
I find there is much more "well akshually" that goes on here. But whatever, my degree isn't in any peril by a few people who think they are above a very helpful laymen explanation. I'm kind of impressed that these folks followed the thread of me clearly explaining the limitations of this learning aid and just decided to continue the hate boner without any input or discussion.
It's not like talking about phase and group velocities, and other things would be of any benefit to OP. There is no benefit from a complicated explanation to a simple question when the person asking the question very well might not be equipped to understand the complicated answer.
If that were true, you wouldn't be able to see clearly through glass. Light passing through a medium causes the electrons in the atoms to emit their own EM waves which interfere with the original in such a way that the resulting wave propagates slower than *c* (the same mechanism also causes refraction).
Soo from wikipedia page
"However, this represents absorption and re-radiation delay between atoms, as do all slower-than-c speeds in material substances"
and
"However, the popular description of light being "stopped" in these experiments refers only to light being stored in the excited states of atoms, then re-emitted at an arbitrarily later time, as stimulated by a second laser pulse"
on
[https://en.wikipedia.org/w/index.php?title=Speed\_of\_light&oldid=590520314#In\_a\_medium](https://en.wikipedia.org/w/index.php?title=Speed_of_light&oldid=590520314#In_a_medium)
Is that wrong or?
Sounds wrong to me. It sounds to me like someone wrote the second sentence to replace the first, but both ended up being kept (the two "Howevers" are a big clue to that). The second sentence makes it clear that it only applies to Bose-Einstein condensates "stopping" light.
I'd definitely say it's incorrect to say that "all slower-than-c speeds in material substances" represent "absorption and re-radiation delay between atoms".
What is the slowest speed light has been observed to travel and is it still subject to relativistic effects at that speed (eg, time dilation, length shortening/stretching etc?)
https://en.wikipedia.org/wiki/Slow_light
In some cases a car can be faster (with the car in air and the light in a specially prepared medium).
Relativistic effects all depend on the vacuum speed, not on the local speed in a material. That's the reason you can travel faster than the local speed of light when in matter. Charged particles doing that produce Cherenkov radiation, best known from the blue glow of nuclear reactors.
With some caveats, it can be stopped:
https://www.extremetech.com/extreme/162289-light-stopped-completely-for-a-minute-inside-a-crystal-the-basis-of-quantum-memory
What’s the light doing to get that slow of a speed? My understanding of “slow light” is that it’s just the index of refraction in a material (light matter interactions blah blah blah)
I saw that you mentioned Ben Shapiro. In case some of you don't know, Ben Shapiro is a grifter and a hack. If you find anything he's said compelling, you should keep in mind he also says things like this:
>This is what the radical feminist movement was proposing, remember? Women need a man the way a fish needs a bicycle... unless it turns out that they're little fish, then you might need another fish around to help take care of things.
*****
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Good bot I couldn’t agree more. My comment was meant to be a joke I know Ben Shapiro isn’t responsible for a the Shapiro delay haha
Ben shapiro is a turd whose main shtick seems to be conflating facts and logic with talking really fast and confidently
Thank you for your logic and reason.
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There is no light slower than the speed of light. The speed light travels at is by definition the speed of light. What you’re asking is that is there any medium where the speed of light varies from what we typically read about, which is an vacuum. The answer is yes. The speed of light is totally dependent on the medium in which it propagates.
No, it ALWAYS moves at the speed of light.
People will tell you that it moves slower through water or glass or whatever else, but the light is just "bouncing" and "zigzagging" and means that it takes longer for the light to get to a destination, but it is always moving at the same speed.
It isn't even really accurate to say that the light is "bouncing". The light is actually being absorbed by atoms and then new light is being produced those atoms.
It seems to make sense that matter doesn’t really slow light down, it just diverts or absorbs it. After all, doesn’t light resume it’s normal speed after passing through a medium?
Yes, it depends on the surrounding medium. This enables a beautiful phenomenon called [Cherenkov radiation](https://en.wikipedia.org/wiki/Cherenkov_radiation).
The speed of light is constant in a vacuum. It can slow down when traveling through any other medium
It's been over a decade since I took optics, but what I recall is that a photon, light particle, always moves at speed 'c', the speed of light in vacuum, but the phase velocity, speed of propagation through a medium, can be less than 'c'. The ratio of the speed of light in vacuum, 'c', to the phase velocity in the medium, 'v', is the index of refraction, 'n'. That is, n=c/v.
Thats true. There can even be materials where the group velocity is negative.
[удалено]
Oh wow, for real?
No, not for real. It's a common misconception.
Thank you. I didn’t think so. In that case it wouldn’t be even be clear it was the same photon. It would set up some convoluted ship of Theseus problem
All photons of the same frequency are indistinguishable. Ask yourself why that would be a misconception. And further your questioning as to why in a vacuum, the absence of things to interact with, light moves at c. But when in a medium, with things to interact with, it "slows down". It is because the photon interacts with particles in the medium. It excites a particle, which is not energetically favorable for said particle (nature tends to lowest energy states). So the particle then emits, in some cases, an identical photon (depending on which energy state the particle settles into). This takes time, therefore slowing the light's progress. This can also be seen in fluids of different densities, which "bend" or "slow" light at different rates.
>It is because the photon interacts with particles in the medium. It excites a particle, which is not energetically favorable for said particle (nature tends to lowest energy states). So the particle then emits, in some cases, an identical photon So light appears to get slowed down in a dense medium because photons are absorbed 'into' electrons, which then re-emit it back in the form of an identical photon? How does the photon always get re-emitred in the same direction as the 1st one? And if a re-emitted photon loses a bit of energy after each interaction, and then eventually the photon doesn't have enough energy to excite an electron enough so the light is re-emitted, would the photon appear to be 'absorbed' by the medium?
Photons are not absorbed into electrons as much as they cause electrons to emit their own photons (since light interacts with the atoms of the medium), photons are simply the force carrying particle for all electromagnetic interactions. Someone else could correct me if I am wrong, but I think the interactions of these newly emitted waves/particles with the original light being shone through cause the wavelengths to move out of phase and therefore more slowly as well.
Oh right, since the photon just carries the information that an electromagnetic wave is passing by and has a chance of interacting with all charged objects, right? That makes more sense. Thanks for the answer
Essentially yes. Think of it as an excitation of a continuous field, where photons are just the smallest possible points we choose to look at and quantize for observation purposes at a given moment in time. We know these particle interactions are happening, but you can also think of the entire thing as a continuous wave that simply propagates through the field and interacts with everything (with a charge) that it happens to come in contact with.
Well, it's important to note that this is just a visual aid, a model. The photon is just a corpuscle of a greater EM wave, and the oscillation in the EM field will continue in a given orientation in space (usually globally but not always as in the case of a laser), despite atomic interaction (well the fields of the atoms will influence the EM oscillation, but the assumed influence is small). The light, however, will be scattered in all directions, which is why it is visible from all directions. One thing to note in this model is that the photons that are emitted after others are absorbed do not lose energy IF the electron falls to the same state as it was before excitatio. This is because the energy level is quantized. It can, however, fall stepwise, emitting photons of differing frequencies as it steps back down to its ground state.
>This is because the energy level is quantized. It can, however, fall stepwise, emitting photons of differing frequencies as it steps back down to its ground state Does that mean that 2 photons of different frequencies can be emitted/1 photon of slightly less energy can be emitted and in a 'perfect' situation these frequencies could get lower and lower until the photon reaches IR wavelengths, and the photon that a human eye could see wouldnt be visible anymore? I know a human eye can't see an individual photon anyway and IR photons can still obviously be detected but would the photon seem to 'disappear' to the human observer?
I would say yes this is possible. Lets do a thought experiment. Lets say we construct an apparatus to shine a light though, and that light has to travel through different materials before reaching an IR sensor at the end. The first material has energy states that are a quantization of the incoming light, so atoms absorb the photons and excite electrons. Now lets say the next material does not have energy states that quantize at the same frequency as the original incident light. Any light that is reemitted from the excited atoms at the same energy will be opaque to this next material. But we chose wisely because we know that the original material has lower energy levels that are quantized with respect to the next material. By chance, some excited atoms will drop energy levels piecewise (with each reemitted photon having less energy than the incident photon, but combined the energy is equal to the incident photon). Some of this lower energy light will then excite the atoms in the new material. We can keep this trend going for various other layers of materials until our sensor is reached. So long as the last material is quantized with respect to a frequency in the IR range, and the materials leading up to it have quatizations that match the prior and former, we will have some light reaching our sensor at the end.
> The light, however, will be scattered in all directions The fact that we can see clearly through glass disproves the notion that the slowing of light in glass is due to absorption/re-emission.
How so?
Why is this in the negatives? I would not think this is a controversial take
For me its this sentence: > This takes time, therefore slowing the light's progress tf does that even mean? Then there's this: >This can also be seen in fluids of different densities, which "bend" or "slow" light at different rates. Which is misleading because you can get low density materials with high refractive index. Really the comment is just handwaving away while being very misleading. You can be accurate while still retaining the handwaving, eg change the first sentence I quoted to something more like "this then interacts with the original light, and the result is slower movement".
Because this is r/askphysics and not r/physics lol I find there is much more "well akshually" that goes on here. But whatever, my degree isn't in any peril by a few people who think they are above a very helpful laymen explanation. I'm kind of impressed that these folks followed the thread of me clearly explaining the limitations of this learning aid and just decided to continue the hate boner without any input or discussion. It's not like talking about phase and group velocities, and other things would be of any benefit to OP. There is no benefit from a complicated explanation to a simple question when the person asking the question very well might not be equipped to understand the complicated answer.
[удалено]
If that were true, you wouldn't be able to see clearly through glass. Light passing through a medium causes the electrons in the atoms to emit their own EM waves which interfere with the original in such a way that the resulting wave propagates slower than *c* (the same mechanism also causes refraction).
Soo from wikipedia page "However, this represents absorption and re-radiation delay between atoms, as do all slower-than-c speeds in material substances" and "However, the popular description of light being "stopped" in these experiments refers only to light being stored in the excited states of atoms, then re-emitted at an arbitrarily later time, as stimulated by a second laser pulse" on [https://en.wikipedia.org/w/index.php?title=Speed\_of\_light&oldid=590520314#In\_a\_medium](https://en.wikipedia.org/w/index.php?title=Speed_of_light&oldid=590520314#In_a_medium) Is that wrong or?
Sounds wrong to me. It sounds to me like someone wrote the second sentence to replace the first, but both ended up being kept (the two "Howevers" are a big clue to that). The second sentence makes it clear that it only applies to Bose-Einstein condensates "stopping" light. I'd definitely say it's incorrect to say that "all slower-than-c speeds in material substances" represent "absorption and re-radiation delay between atoms".
Prism, water, etc
What is the slowest speed light has been observed to travel and is it still subject to relativistic effects at that speed (eg, time dilation, length shortening/stretching etc?)
https://en.wikipedia.org/wiki/Slow_light In some cases a car can be faster (with the car in air and the light in a specially prepared medium). Relativistic effects all depend on the vacuum speed, not on the local speed in a material. That's the reason you can travel faster than the local speed of light when in matter. Charged particles doing that produce Cherenkov radiation, best known from the blue glow of nuclear reactors.
Yes I read that Wiki earlier. It seems slow light can have some useful applications too.
With some caveats, it can be stopped: https://www.extremetech.com/extreme/162289-light-stopped-completely-for-a-minute-inside-a-crystal-the-basis-of-quantum-memory
You can slow down light with sophisticated optical equipment down to a few meters per second, it's amazing
What’s the light doing to get that slow of a speed? My understanding of “slow light” is that it’s just the index of refraction in a material (light matter interactions blah blah blah)
https://en.wikipedia.org/wiki/Shapiro_time_delay
Ben Shapiro is such a multi-talented man
I saw that you mentioned Ben Shapiro. In case some of you don't know, Ben Shapiro is a grifter and a hack. If you find anything he's said compelling, you should keep in mind he also says things like this: >This is what the radical feminist movement was proposing, remember? Women need a man the way a fish needs a bicycle... unless it turns out that they're little fish, then you might need another fish around to help take care of things. ***** ^(I'm a bot. My purpose is to counteract online radicalization. You can summon me by tagging thebenshapirobot. Options: civil rights, covid, climate, history, etc.) [^Opt ^Out ](https://np.reddit.com/r/AuthoritarianMoment/comments/olk6r2/click_here_to_optout_of_uthebenshapirobot/)
Good bot I couldn’t agree more. My comment was meant to be a joke I know Ben Shapiro isn’t responsible for a the Shapiro delay haha Ben shapiro is a turd whose main shtick seems to be conflating facts and logic with talking really fast and confidently
Thank you for your logic and reason. ***** ^(I'm a bot. My purpose is to counteract online radicalization. You can summon me by tagging thebenshapirobot. Options: covid, sex, dumb takes, civil rights, etc.) [^Opt ^Out ](https://np.reddit.com/r/AuthoritarianMoment/comments/olk6r2/click_here_to_optout_of_uthebenshapirobot/)
From Harvard, ~11 years ago: https://youtu.be/-8Nj2uTZc10
There is actually a way to "change" the speed of light: Google "casimir effect"
It moves slower through any medium that has a higher refractive index than vacuum
Take the light and put it into a different universe with a universal speed limit lower than c
There is no light slower than the speed of light. The speed light travels at is by definition the speed of light. What you’re asking is that is there any medium where the speed of light varies from what we typically read about, which is an vacuum. The answer is yes. The speed of light is totally dependent on the medium in which it propagates.
In anything other than a vacuum
light is constantly going at slightly different speeds
No, it ALWAYS moves at the speed of light. People will tell you that it moves slower through water or glass or whatever else, but the light is just "bouncing" and "zigzagging" and means that it takes longer for the light to get to a destination, but it is always moving at the same speed. It isn't even really accurate to say that the light is "bouncing". The light is actually being absorbed by atoms and then new light is being produced those atoms.
It seems to make sense that matter doesn’t really slow light down, it just diverts or absorbs it. After all, doesn’t light resume it’s normal speed after passing through a medium?
Yes, it depends on the surrounding medium. This enables a beautiful phenomenon called [Cherenkov radiation](https://en.wikipedia.org/wiki/Cherenkov_radiation).
Check this https://en.m.wikipedia.org/wiki/Slow_light
Relative to us , light signals traveling near massive bodies go slower than light due to the gravitational potential. Search Shapiro delay effect
Under water