We don’t have a good enough understanding of light to completely answer that question. I say that after working with photons as my main livelihood for more than 45 years as an optical physicist.
Light behaves like waves or like particles, but not like particles people understand. One way to describe light is with waves following Maxwell’s equations. Another way is with Quantum Electrodynamics (QED). In recent decades, teaching QED has become more popular, but generally it is taught in a completely incorrect (or at least very incomplete) manner. It is simplified and made very misleading for unde
We don’t have a good enough understanding of light to completely answer that question. I say that after working with photons as my main livelihood for more than 45 years as an optical physicist.
Light behaves like waves or like particles, but not like particles people understand. One way to describe light is with waves following Maxwell’s equations. Another way is with Quantum Electrodynamics (QED). In recent decades, teaching QED has become more popular, but generally it is taught in a completely incorrect (or at least very incomplete) manner. It is simplified and made very misleading for undergraduates and non-optical physicists.
It is taught in a way to give people a picture in their heads that photons rush in-between atoms at a speed of c, but then get absorbed by the electrons in atoms and get re-emitted. Let me explain why this picture is totally false and should never be taught to students.
First it gives the idea that photons have to collide with an electron to interact with it. This is false.
Second, it gives the idea that photons are literally absorbed by electrons which go into a higher energy state for some period of time and then re-emit the photons. This is totally false. There are cases where a photon is absorbed by an electron near the resonance of the oscillator, but that photon is rarely re-emitted at the same wavelength and almost never in the same direction, phase and polarization as the original photon. These parameters are randomized in absorption and spontaneous re-emission. The only time you get a remotely similar behavior to absorption and re-emission is stimulated re-emission in a laser gain medium.
Third, it gives the idea that the space between atoms is the same as a vacuum. The electromagnetic environment between atoms is nothing of the sort.
Fourth, it gives the idea that photons are tiny particles that interact only at very short ranges. In fact, photons are not localized and “take every possible path simultaneously.” Somehow, a single photon interferes with itself even though the two paths are many miles apart. You can only conclude that the photon does not act like a single particle when it propagates through space or matter.
Fifth, it gives the idea that a photon can only interact with the electrons in an atom by being totally absorbed. In fact, photons can cause more subtle changes in the motion of electrons in an atom with much smaller amounts of energy than a photon quantum.
In other words, for all intents and purposes, photons act like waves that are quantized. Feynman and his colleagues spent a lot of time coming up with mathematical rules to make photons behave like waves. They concocted rules like: “Each photon interacts and is absorbed (partially) by every electron in the substance up to an infinite number of times simultaneously.” When all is said and done, you get Maxwell’s equations with the interpretation that the square of the electric field is the probability of finding the photon.
So how does the wave slow down? Well, it is actually forcing all those electrons in the material to oscillate, but because the electrons lag a little bit, the phase of the wave gets very slightly delayed, and the wavefront propagates more slowly in the presence of those electrons. Some of the wave’s energy is temporarily stored in the oscillation of the electrons in the atoms. As the wave exits the medium, the energy stored in the oscillating electrons is restored to the wave.
I said we don’t understand this well enough to thoroughly explain it. People have a problem with how the oscillating electrons can make new photons when each one of them does not have enough energy to make a photon, or how they can oscillate when they don’t absorb enough energy each to equal a whole photon. And while the electrons oscillate, where does the driving energy come from if the photon is still intact? To me, this points out some issues with the idea that light can be totally explained with photons. You have to resort to partial photons and cumulative probabilities to explain the simplest phenomena.
In reality, there is no experiment that can be devised to answer these questions. We don’t know the exact mechanism of interaction and nature has conspired to keep that forever hidden from us. So all we can do is devise explanations that each have their limitations and inconsistencies. QED addresses this by saying the photons are partially absorbed and partially re-emitted. It works mathematically, but nobody knows what that means. Wave theory, of course, fails to explain quantization when a photon actually is absorbed.
If you accept QED as the “correct” explanation, you must consider that each photon takes all possible paths, interacts with all possible electrons, and the result is an infinite sum of all the possible interactions, each one multiplied by its respective probability. That makes the idea of a photon a lot different from what most people think of when they say “photon”.
The first ever photograph of light as both a particle and wave
Free Feynman Lectures (printed material)
The Origin of the Refractive Index
Refractive Index of Dense Materials
https://archive.org/details/QuantumElectrodynamics
Other answers on Quora by Bill Otto
Bill Otto's answer to How do photons bounce off a mirror?
Disclaimer:
My answer does not necessarily reflect the views of the Boeing Company or the US Department of Defense and is intended for informational purposes only.
This is a fact that absolutely fascinates me. Photons do NOT accelerate, but they do change velocities!
The equation for the energy of a photon is simply:
[math]E = hf[/math]
Nowhere in this equation does the index of refraction occur, and the frequency of the photon remains constant. So the energy of a photon remains constant as well.
This means it takes absolutely no energy to slow down a photon to a complete standstill, or to allow it to speed up again to the speed of light!
Now the interesting question then becomes not about energy, but momentum.
[math]p = \dfrac{Ev}{c^2} = \dfrac{hf}{nc}[/math]
Momentum is a function of
This is a fact that absolutely fascinates me. Photons do NOT accelerate, but they do change velocities!
The equation for the energy of a photon is simply:
[math]E = hf[/math]
Nowhere in this equation does the index of refraction occur, and the frequency of the photon remains constant. So the energy of a photon remains constant as well.
This means it takes absolutely no energy to slow down a photon to a complete standstill, or to allow it to speed up again to the speed of light!
Now the interesting question then becomes not about energy, but momentum.
[math]p = \dfrac{Ev}{c^2} = \dfrac{hf}{nc}[/math]
Momentum is a function of index of refraction. Normally a change in momentum requires a force applied over a distance. If so, that seems to imply there is an infinite force that acts on a photon as it enters and leaves a medium.
If so where does that force come from? Answer: There is no force. This is an impulse, not a force. Meaning a sudden change in momentum. To understand why, we have to understand what causes light to slow down to begin with.
Light propagates as a wave. Waves are slowed down by both virtual and real interactions. The deeper the water the faster the surface wave. Even though the top of a water wave does not interact with the bottom of the ocean, the fact that some parts of the wave do interact with the bottom is enough to slow down the whole wave. The shallower the water, the larger the portion of the wave that can interact, and the more the whole wave slows down.
Likewise, some of the light passing through a medium such as glass will interact with the glass. That is why we see scattering and other such effects. This interaction is enough to slow down the average velocity for the whole wave. Whether or not an individual photon has interacted with the glass, the potential for interaction is sufficient to slow down the wave. So even for light travelling well less than the mean free path distance through a material the whole wave will slow down.
If light slows down as the result of interactions or potential interactions it is not hard to see where the impulse comes from. Each individual interaction can be approximately modeled as an elastic collision. In an elastic collision momentum is transferred but conserved. So if light was purely particles, you would have a photon bouncing around between atoms before making its escape. And while it is bouncing around those atoms the system of atoms is holding onto part of the momentum. Of course, light travels as a wave, so we don’t normally have photons playing pinball as they pass through glass. But the motion of the wave is an integration of all those possible pinball paths that could be followed.
As soon as the wave leaves the medium, there is no more pinball action possible, so the wave must leave with all the momentum it came in with. Unless a measurable effect happened such as reflection or scattering to allow a permanent transfer of momentum.
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The thing to understand is that the effect where the speed of light in materials is lower is a macroscopic effect that comes from the sum of a bunch of complicated paths of individual photons on the micro level. It's an aggregate effect. Individual photons are not exactly slowed down. But the thing is that in materials, photons, which generally move in a straight(est) line path, don't always have
The thing to understand is that the effect where the speed of light in materials is lower is a macroscopic effect that comes from the sum of a bunch of complicated paths of individual photons on the micro level. It's an aggregate effect. Individual photons are not exactly slowed down. But the thing is that in materials, photons, which generally move in a straight(est) line path, don't always have a free path. Matter gets in the way. So some photons pass right through, while others bump into atoms and get deflected or absorbed/re-transmitted. When you add that up, some heap of photons you send through the material will, on aver...
When photons travel through a medium like glass, they interact with the atoms in that medium, which causes their effective speed to be reduced. This reduction in speed is not because the photons themselves lose energy; rather, it is due to the time it takes for them to be absorbed and re-emitted by the atoms in the glass.
When photons exit the glass and enter a vacuum, they resume their speed of light, [math]c[/math]. The increase in speed does not require an external source of energy because the photons retain their energy throughout the process. The energy of a photon is related to its frequency (or wavel
When photons travel through a medium like glass, they interact with the atoms in that medium, which causes their effective speed to be reduced. This reduction in speed is not because the photons themselves lose energy; rather, it is due to the time it takes for them to be absorbed and re-emitted by the atoms in the glass.
When photons exit the glass and enter a vacuum, they resume their speed of light, [math]c[/math]. The increase in speed does not require an external source of energy because the photons retain their energy throughout the process. The energy of a photon is related to its frequency (or wavelength) by the equation:
[math]E = h \nu[/math]
where [math]E[/math] is the energy of the photon, [math]h[/math] is Planck's constant, and [math]\nu[/math] is the frequency of the photon.
As photons transition from glass to a vacuum, their energy remains constant since the frequency does not change when they exit the medium. The increase in speed is a result of the absence of interactions with the medium, allowing them to travel freely at the speed of light in a vacuum. Thus, they do not gain additional energy; they simply transition back to their original speed.
The premise of your question is false! Light always propagates through space at the speed of light. What we perceive as a “slower velocity” when light propagates through any matter is actually an elongated path. Each particle of the material has a mass and/or charge that distorts the electromagnetic field according to Einstein’s field equations. These field distortions force the light to propagate around all the particles in the electromagnetic field along a “winding path.” The refractive index is a measurement of the distortion a material causes in the electromagnetic field. Since light doesn
The premise of your question is false! Light always propagates through space at the speed of light. What we perceive as a “slower velocity” when light propagates through any matter is actually an elongated path. Each particle of the material has a mass and/or charge that distorts the electromagnetic field according to Einstein’s field equations. These field distortions force the light to propagate around all the particles in the electromagnetic field along a “winding path.” The refractive index is a measurement of the distortion a material causes in the electromagnetic field. Since light doesn’t actually “slow down” on the way into the material, it doesn’t have to “speed up” on the way out. When the light leaves the material, it simply resumes it’s ideal path through space.
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That’s not the right way to look at it. What happens slower than the speed of light is the progress of net energy through the medium. Think about what you’re actually observing in such a situation. You have some sort of a detector on the far side of the glass, and you measure the arrival time of energy at that location. You are not “watching” the photons as they move from their source toward your
That’s not the right way to look at it. What happens slower than the speed of light is the progress of net energy through the medium. Think about what you’re actually observing in such a situation. You have some sort of a detector on the far side of the glass, and you measure the arrival time of energy at that location. You are not “watching” the photons as they move from their source toward your detector. Therefore, it’s basically illegitimate to talk about their trajectory during that journey. All you know is that measurable energy arrives at the detector later than it would if there was nothing but vacuum along the photons’ paths. You know nothing about any individual photon prior to it reaching you.
People talk a lot about this scenario, bu...
Photons travel at the speed of light (c) in a vacuum. When light enters a medium such as glass, the light beam (composed of photons) slows down in forward speed (v), the wavelength (λ) is shorter but the frequency (υ) remains the same. Since the frequency stays contant in the medium the light does not lose energy according to E=hυ. The forward speed of the light beam is reduced depending on the index of refraction (n) of the medium. For flint glass, n=1.62, where n in a vacuum is unity, the velocity of light in glass is n=c/v, or v = c/n. Here the index of refraction is defined as the speed of
Photons travel at the speed of light (c) in a vacuum. When light enters a medium such as glass, the light beam (composed of photons) slows down in forward speed (v), the wavelength (λ) is shorter but the frequency (υ) remains the same. Since the frequency stays contant in the medium the light does not lose energy according to E=hυ. The forward speed of the light beam is reduced depending on the index of refraction (n) of the medium. For flint glass, n=1.62, where n in a vacuum is unity, the velocity of light in glass is n=c/v, or v = c/n. Here the index of refraction is defined as the speed of light in a vaccum divided by the speed of light in a medium. This equation can be expanded using (c=λυ), λυ/λ’υ’ = c/v, (here υ=υ’), so λ/λ’=c/v, or λ/λ’ = n, λ’=n/λ. The wavelength of the light beam in the medium is shorter by n/λ. When light emerges from the medium back into a vaccum, the state of the light is the same as before it entered except for a decrease in intensity according to Beer’s Law, I = Io[exp]-cz, where Io = original intensity, c = total attenuation coefficient, where c = a+b, a = absorption, b = scattering, z = thickness of the medium, 1/c= one attenuation length. In water, n=1.33, it becomes difficult to see another diver after four attenuation lengths. In very clear water where c=0.1, four attenuation lengths is 4/0.1 = 40 meters, see Jerlov.
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-Light changes wavelength as it passes from one medium to another because it changes speed as it enters the new medium.
Since acceleration is a change of speed, and since light travels at speed 0.65 c in glass and then changes to speed c in air once it exits the glass, you could technically say that light accelerates. But this is very misleading. The term "acceleration" is usually used to describe how objects with mass change speed in response to a force. Light does not change speed because a force speeds it up.
So it changes speed because it is in a different medium,this makes the most sense if
-Light changes wavelength as it passes from one medium to another because it changes speed as it enters the new medium.
Since acceleration is a change of speed, and since light travels at speed 0.65 c in glass and then changes to speed c in air once it exits the glass, you could technically say that light accelerates. But this is very misleading. The term "acceleration" is usually used to describe how objects with mass change speed in response to a force. Light does not change speed because a force speeds it up.
So it changes speed because it is in a different medium,this makes the most sense if you look at light as a wave and not a particle,for waves, the speed of propagation is determined by the properties of the medium or field they are traveling through (i.e. how well the medium snaps back), and not by how much of a force is applied to speed up the object,waves cannot be made to go faster by pushing on them.
Because light is a wave, it instantaneously switches speed the moment it leaves the glass and enters air,you can't think of it a particle suddenly snapping to a faster speed.
Rather then it is a wave that starts waving differently the moment it enters a new medium,as an instantaneous change in speed corresponds to an infinite acceleration,as you can see, applying the concept of acceleration to waves is not very meaningful, even though speeds changes
So we can say the speed of the light wave changes as instantaneously as the transition from medium to vacuum does.
As light travels from a less dense medium to a more dense medium (like from air to glass), its velocity decreases, so the wavelength increases. It is this change that causes light to bend
Now the speed of light doesn't actually change, because the speed of light is a fundamental constant,however, it does take longer to pass through a medium with a higher refractive index,for this explanation, we can think of light as a particle.
Refractive index is determined by a material's density,that when light passes through a material of high density (glass for instance), it collides with and is scattered from the glass molecules.
So the light doesn't actually take a straight path through the material, it bumps from molecule to molecule,in a dense material, there are more particles in a smaller volume so it gets bumped around more, and ends up taking a longer distance to the other end of the medium as compared to a less dense material.
But the speed of the light from molecule to molecule is always c, but we like to simplify it to say that it travels in the shortest distance, but at a different speed.
A.: There is no energy loss or gain associated with the change of speed.
The presumption* that light “hits the (squealing) brakes” to slow down in media, and then needs to “put the pedal to the metal” to speed up as it exists the optically denser media reveals the faults of the caricature(s) many nurture, not the actual physics behind the phenomenon.
The actual physical phenomenon is …complex.
(What else did you expect, of even just one photon interacting with a medium that consists of a hugigantimongous number of charged particles?!)
#1. Light (in fact all electromagnetic radiation) is the space
A.: There is no energy loss or gain associated with the change of speed.
The presumption* that light “hits the (squealing) brakes” to slow down in media, and then needs to “put the pedal to the metal” to speed up as it exists the optically denser media reveals the faults of the caricature(s) many nurture, not the actual physics behind the phenomenon.
The actual physical phenomenon is …complex.
(What else did you expect, of even just one photon interacting with a medium that consists of a hugigantimongous number of charged particles?!)
#1. Light (in fact all electromagnetic radiation) is the spacetime-changing electromagnetic quantum field, and photons are the quanta (“packets” of minimal (Hamilton's) action, [math]=1\,{\cdot}\,2\pi\hbar[/math]) of such change. Like any other spacetime-change in any field, quanta can take continuously infinite many different forms/shapes; quanta are merely small —in the amount of their (Hamilton's) action, not necessarily spatial or temporal “size.”
#2. Electrons† in a medium are quanta of the electron quantum field, i.e., the “packets” of change in that field with minimal (Hamilton's) action, [math]=1\,{\cdot}\,2\pi\hbar[/math]. Owing to Pauli's exclusion principle, no two electrons can be in the same state, but the overall state of the ensemble a hugigantimongous number (on the order of Avogadro’s number, [math]{\sim}\,10^{23}[/math]) of electrons is nevertheless in some of multi-particle states. (If you really prefer, think of the myriad of electrons as being entangled.)
Therefore: When a photon (remember: it can have the shape/form of anything, from a spatially perfectly localized “point-particle,” to a perfect-wavelength “plain wave,” or anything else) hits the medium (in which the collective of charged particles can have all kinds of shapes/forms), any of umteenillion different processes are possible:
- from a “(spatially) point-like” photon being absorbed by a “(spatially) point-like” electron, which thereby jumps into a higher-energy state. Even so, the electron may have been originally bound within an atom, or perhaps in a molecule, or even more “diffusely” in a macro-molecule, or…
- to a “(spatially) point-like” photon interacting with the collective of electrons that roam the bulk of the medium fairly freely as part of the Fermi gas, in which case the energies of the gas’ quantum states are discrete but so dense that they are practically continuous — albeit with significant gaps…
- to a perfect-wavelength “plain wave” photon interacting with a “(spatially) point-like” electron…
- to a perfect-wavelength “plain wave” photon interacting with the collective Fermi gas of electrons…
- to anything “in-between” those particular extreme cases (and corresponding caricatures)…
- …oh, and one should also include the interaction(s) with any and all kinds of virtual particles that have any (however small) probability of turning up in the medium…
- …and then realize that there may well occur cascading interactions of all of the above-outlined kinds…
…and there’s no experiment to identify which of those particular, possible processes “in fact” happens. (Every possibleintervention designed to identify which of those particular, possible processes “in fact” happens changes the outcome to what we are not trying to examine.) Whence quantum field theory instructs to include (a priori) all of them in your consideration. (This is sometimes encapsulated in the “totalitarian principle”:
“Everything not forbidden is compulsory.” [promoted by Murray Gell-Mann, from T.H. White, The Once and Future King, 2nd ed., Ace Books, 1987.]
→ my answers to “How can you describe quantum physics?” and “What is the basis of quantum theory?”)
Of course, we “organize” those “particular, possible processes” in order of the magnitude of their contribution to the particular observable(s) we are interested in computing, and include those that contribute most for an initial estimate. Then we add the “next-order” contributions to correct the numerical answer(s), and so on. Since they all contribute (more or less), they can all be thought of “in fact,” happening, and simultaneously.‡
In the previous paragraph, there is no a priori reason that the “dominant” contribution to any particular phenomenon of one’s interest would turn out to be any one of the above possibilities.
That is why practically none of the above possibilities can be a priori guaranteed to “dominate” in any particular phenomenon.
For the interaction of even a single photon with a medium that consists of a hugigantimongous number of charged particles, the caricature of a “lone point-particle photon” interacting with a “lone point-particle electron” is… bad. The caricature of a “lone plain-wave photon” interacting with a “lone plain-wave electron” is… better. (But is nevertheless just a caricature.)
That’s all.
For my own humble part, it seems to me that when erudite and actually practicing physicists say that “no one actually understands what this means,” they mean that it cannot be ’splained in dumbed-down caricature terms. Or, perhaps, they forget that all those various caricatures are simply maps to the actual territory, not the territory itself: they are descriptions of the physical phenomenon, not the phenomenon itself. → “When a photon hits a mirror and goes from v=c to v=-c is there a moment when v=0? Does the mass of the photon decrease during this time?”
* Yes, the ‘squealing brakes’ and ‘pedal to the metal’ are my exaggerations, meant to highlight the silliness of caricatures in general. In turn, every scientific model employs caricatures for ease of communication; what gets lost in the translation is the fact that actually working physicists know that those are rough, sketchy, mnemonic caricatures, only meant to highlight one or another feature of the “real thing.”
† Every medium also contains other charged particles, not just electrons. Notably, every atom has a (positively charge) nucleus, which also can interact electromagnetically. Yes, atomic nuclei are many thousands of times more massive than any individual electron, so interaction with them is harder, but not absent either. Moreover, just as practically no electron is isolated in any medium but is part of a collective of Avogadro’s numbers of them, so is no atomic nucleus in any medium isolated: they (together with a partial complement of “their” closely bound electrons, forming ions) are in variously coupled networks with other ions in the medium, forming a lattice, a collage of variously oriented and sized lattice-domains, and amorphously random hodgepodge of micro-domains…….
‡ If it makes you happier to think of this in terms of “reality” functioning like a parallel-processing computer, with each of the “particular, possible processes” being a “thread” of that parallel computation, who am I to dissuade you?
To understand it correctly you need to understand what the “photon” is. When you truly understand it, you know that the question isn’t meaningful.
In his book “QED: The strange theory of Light and Matter”, Richard Feynman taught us how to use the idea of photon correctly, at least conceptually. Essentially, from a starting configuration (e.g., a photon is just emitted from a light source) to an ending configuration (e.g., a photon is just absorbed by the electron of an atom), you find all the possible intermediate configurations that this can happen. Each of them will involve a time delay and a
To understand it correctly you need to understand what the “photon” is. When you truly understand it, you know that the question isn’t meaningful.
In his book “QED: The strange theory of Light and Matter”, Richard Feynman taught us how to use the idea of photon correctly, at least conceptually. Essentially, from a starting configuration (e.g., a photon is just emitted from a light source) to an ending configuration (e.g., a photon is just absorbed by the electron of an atom), you find all the possible intermediate configurations that this can happen. Each of them will involve a time delay and a distance, which will “turn a dial” of the photon, while diminishing the contribution (attenuation). They might also involve the photon absorbed by a particle and re-emitted. This again involves a turn of dial and an attenuation. Because there are many ways for this to happen, you need to find all of them and add up all the attenuated dials like vectors (they are actually complex numbers). At the end you find the total dial, and the square of length of this dial is proportional to the probability that the ending configuration occurs.
Now what does it really mean? It means the whole idea of photons is not localized at all! You cannot ask “where the photon is 1ns before it is absorbed”. It was everywhere.
What physical concept can be “everywhere” at once? It is a wave. In other words, photon is not a “particle” at all. It is a wave.
But it is a sort of wave rather different from our everyday waves. When you think of waves like water waves, the effect of the wave will be felt everywhere that the wave goes. Not for photons. At the end only one place will feel the photon. At all other places, it behaves as if the photon has not been there from the beginning. In this regard it is very similar to a particle: at the end it interacts only with one macroscopic object. So the wave is not the normal type of wave. It is a “probability wave”.
So “photons” should better be considered as “suddenly popping up” in the photon field, taking up one unit of energy from the photon field, where the amount depends on the frequency of light involved. Because it “suddenly pops up”, you cannot say it “accelerates”. To accelerate, there must be a continuous presence of a particle. We don’t have one for photons: it just pops up suddenly and interacts with an electron.
Perhaps the figure below will help you understand what’s going on. The change of angle of the light wave front corresponds to the change of velocities between the two media. In this case the light ray that penetrates the glass slows down to about 80% of the velocity in air. But the wave fronts propagate in a way to stay parallel to each other.
With this figure, you can easily prove that
[math]\dfrac{v_1}{v_2}=\dfrac{\sin{\theta_1}}{\sin{\theta_2}}[/math]
and the time delay generated by the propagation in the glass is equal to
[math]t=\dfrac{a}{v_1}\dfrac{\sin{(\theta_1-\theta_2)}}{\sin{\theta_2}}[/math]
Perhaps the figure below will help you understand what’s going on. The change of angle of the light wave front corresponds to the change of velocities between the two media. In this case the light ray that penetrates the glass slows down to about 80% of the velocity in air. But the wave fronts propagate in a way to stay parallel to each other.
With this figure, you can easily prove that
[math]\dfrac{v_1}{v_2}=\dfrac{\sin{\theta_1}}{\sin{\theta_2}}[/math]
and the time delay generated by the propagation in the glass is equal to
[math]t=\dfrac{a}{v_1}\dfrac{\sin{(\theta_1-\theta_2)}}{\sin{\theta_2}}[/math]
They don't accelerate. Photons travel at the speed of light. The fact is that the photons coming out of the other side of the glass are not the same ones that went in to the glass.
As photons hit the glass, they scatter off the atoms in the glass -- or rather, they are absorbed by the glass atoms, inducing a vibration of the atom, and then the vibration causes a new photon to be emitted in a random direction. There's a delay between when the photon is absorbed and when the new one is emitted.
Potential energy is stored in the glass.
As the wave enters the glass, positive charged particles are separated temporarily from negative charged particles. So potential energy is stored in the electric field between the separated charges.
The electrically charged particles vibrate like waves. The separated charges are a bit like sound waves. They also have a particle-like natures. They are called phonons.
The the electromagnetic wave and the polarization field are traveling together. Solid state physicists sometimes refer to this combined wave as a photon-polariton.
The photon in glass can be pic
Potential energy is stored in the glass.
As the wave enters the glass, positive charged particles are separated temporarily from negative charged particles. So potential energy is stored in the electric field between the separated charges.
The electrically charged particles vibrate like waves. The separated charges are a bit like sound waves. They also have a particle-like natures. They are called phonons.
The the electromagnetic wave and the polarization field are traveling together. Solid state physicists sometimes refer to this combined wave as a photon-polariton.
The photon in glass can be pictured as being ‘dressed’ with the phonon from the glass. The electromagnetic wave and the phonon are traveling together. The photon-polariton travels slower than a photon in vacuum, but faster than the phonon. The photon-polariton travels at a speed c/n, where n is the index of refraction for glass. Note that n in glass is always greater than 1.
This combined particle is very different from the photons described in Einstein’s articles. Einstein’s photons travel completely in a vacuum.
There are no electric charges to separate outside the glass. So there are no phonons to dress the photon. So the photon in a vacuum travels at one speed called ‘c’. The index of refraction for a vacuum is precisely 1.
Solid state physics does not violate relativity, by the way. A photon is not precisely the same as a photon-polariton. A true photon has to travel in vacuum.
Nothing because the speed does not change.
You are thinking of the photon as a particle but, for this, it behaves as a wave.
And waves propagate as fast as their “medium” allows them.
In the glass is about 2/3 than in vacum and thats it. From the point of view of the photon the speed is never reduced. (it is still going as a way at the top speed the light allows on that medium).
It may even be possible that medium every action relative at the relative still applies because the photons are still going at the speed of light.
It is NOT comparable to a bullet that is fired at the air or below a pool. A
Nothing because the speed does not change.
You are thinking of the photon as a particle but, for this, it behaves as a wave.
And waves propagate as fast as their “medium” allows them.
In the glass is about 2/3 than in vacum and thats it. From the point of view of the photon the speed is never reduced. (it is still going as a way at the top speed the light allows on that medium).
It may even be possible that medium every action relative at the relative still applies because the photons are still going at the speed of light.
It is NOT comparable to a bullet that is fired at the air or below a pool. And that seems what you are thinking.
If that were, then photons will decay in their speed, but they do not. In any way they decay on “how many are received on the other side. because some are trapped inside the glass by the reflection or absorption. But the speed is constant.
To give a full answer to this question would need an in depth explanation of Quantum Electrodynamics but Richard Feynmann gave a very good summary in his short, brilliant book QED.
Essentially, in a Quantum Field theory a photon doesn’t have a fixed speed but a probability of being in one place at one moment and at another place at another moment. This probability is calculated by adding up all the probabilities associated with every path the photon could take between the two points and in a vacuum this turns out to give the answer that extremely probably the photon will travel in a straight li
To give a full answer to this question would need an in depth explanation of Quantum Electrodynamics but Richard Feynmann gave a very good summary in his short, brilliant book QED.
Essentially, in a Quantum Field theory a photon doesn’t have a fixed speed but a probability of being in one place at one moment and at another place at another moment. This probability is calculated by adding up all the probabilities associated with every path the photon could take between the two points and in a vacuum this turns out to give the answer that extremely probably the photon will travel in a straight line at the ‘speed of light’.
However, in glass, there are many more possible paths, given by the possibility of the photon being absorbed by an electron in the glass and then the electron emitting a photon etc. The result of adding up all the probabilities for these extra paths results in the measured speed being less than c.
In glass, a photon enters the glass and a photon is measured having travelled through the glass, but there is no way of saying that this is the ‘same photon’. Many interactions may have taken place in between the measurements where a photon is absorbed and a photon is emitted. The end result is that we measure a photon and have the illusion is that it is the same photon that entered the glass travelling at less than the vacuum speed of light.
Every kind of wave behaves that way.
A wave’s frequency remains constant to preserve continuity of the wave at each interface. This means that when a wave exits a slower medium, it’s original speed and wavelength must be restored to it.
But what about conservation of energy? Some of the energy is reflected internally. But the frequency can’t vary, and therefore the speed and wavelength must be completely restored. Therefore the only way energy can be reduced is by a reduction of amplitude. In the case of light, it means some photons are reflected and some are not. Individual photons don’t conser
Every kind of wave behaves that way.
A wave’s frequency remains constant to preserve continuity of the wave at each interface. This means that when a wave exits a slower medium, it’s original speed and wavelength must be restored to it.
But what about conservation of energy? Some of the energy is reflected internally. But the frequency can’t vary, and therefore the speed and wavelength must be completely restored. Therefore the only way energy can be reduced is by a reduction of amplitude. In the case of light, it means some photons are reflected and some are not. Individual photons don’t conserve energy where there is partial reflection, but the proportion of reflected photons conserves energy statistically if there are sufficiently many photons.
Without going into the details is how light interacts with different transparent media, one way to think about his problem is that the energy associated with light is not associated with its speed, but rather with its frequency. Photons are massless, so their energy is not in the form of a classical “kinetic energy” which does depend on the particle’s speed. The energy of each photon is proportional to its frequency which is not dependent on the medium it is in.
Since the speed of light in a transparent medium is its speed in a vacuum divided by the index of refraction of the medium, the speed
Without going into the details is how light interacts with different transparent media, one way to think about his problem is that the energy associated with light is not associated with its speed, but rather with its frequency. Photons are massless, so their energy is not in the form of a classical “kinetic energy” which does depend on the particle’s speed. The energy of each photon is proportional to its frequency which is not dependent on the medium it is in.
Since the speed of light in a transparent medium is its speed in a vacuum divided by the index of refraction of the medium, the speed a wavefront propagates through a transparent medium like water or glass is indeed less than that in air. And then when it re-emerges in air after being transmitted through water or glass, it is at the appropriate speed in that medium. But the photon energies associated with the light does not depend on that speed, but rather on the frequency of the light. And the associated frequency does not change as it goes from one medium to another, so light does not lose or gain energy in the process. (Notice that some photons can be absorbed by a transparent medium, so the total amount of energy transmitted can be reduced by some transparent medium, but that is not what is being asked about. In that case, the energy of each photon that is transmitted has remained unchanged.)
Light exists only as a wave traveling locally at c. In a transparent medium it may travel at c momentarily between periods of absorption/emission by the atoms constituting the material. Macroscopically light thus appears to be moving slower in a transparent medium. At the final reemission light leaving the material is emitted at c just as at every intermediate emission. Again, light has no existence except as waves moving invariantly at c.
In dispersive materials the periods of absorption/emission vary between different frequencies. Prisms are made of such materials.
In birefringent materials th
Light exists only as a wave traveling locally at c. In a transparent medium it may travel at c momentarily between periods of absorption/emission by the atoms constituting the material. Macroscopically light thus appears to be moving slower in a transparent medium. At the final reemission light leaving the material is emitted at c just as at every intermediate emission. Again, light has no existence except as waves moving invariantly at c.
In dispersive materials the periods of absorption/emission vary between different frequencies. Prisms are made of such materials.
In birefringent materials the periods of delay depend on the polarization of absorbed light.
In fluorescent and phosphorescent materials light absorbed in one frequency range is emitted at a lower frequency. Light shifted out of the UV range into our visual range produces strikingly vivid reemissions in some materials, and with extended delays that can persist for hours.
The common idea that photons slow down when passing through transparent media is a quick and easy, if somewhat misleading, way of explaining refraction. In reality, photons are massless and cannot accelerate or decelerate. The reason they appear to speed up again when exiting some medium is because they never really slowed down to begin with. The explanation of what causes photons to appear to slow down is a bit complicated; the oscillating atomic electric fields of the molecules of the medium lengthen the path of the photons. Imagine a ship on water that can only go one speed and can only go
The common idea that photons slow down when passing through transparent media is a quick and easy, if somewhat misleading, way of explaining refraction. In reality, photons are massless and cannot accelerate or decelerate. The reason they appear to speed up again when exiting some medium is because they never really slowed down to begin with. The explanation of what causes photons to appear to slow down is a bit complicated; the oscillating atomic electric fields of the molecules of the medium lengthen the path of the photons. Imagine a ship on water that can only go one speed and can only go forward; on smooth water, the ship will take the least amount of time to get from point A to point B but when there are waves on the water, the ship will take more time due to the vertical component of the waves lengthening the ship’s path. Now, this analogy is flawed because the ship has no effect on the water or the waves, but photons are a bit more interactive with those oscillating electric fields, which is why higher energy photons will refract even more than lower energy photons, which explains rainbows (prismatics).
No acceleration is required, because the photon is massless.
But what really happens is photons always travel at the speed of light. It's the light wave which ends up propagating slower, as an ensemble of photons having complex interactions with the glass (collisions, absorption, emission).
I spent a long time analyzing this situation especially in relation to how momentum is conserved and the kinds of forces exerted in the process. It is now on the arxiv https://arxiv.org/pdf/1406.5123.pdf until I have time to pursue publication. There is also a second chapter to this article I need to finish first. The short answer is that the light is not speeding up or slowing down but it is getting partially absorbed and transformed into energy and momentum in the medium. There is a standing wave component that is generated in the medium despite the fact the incoming and outgoing waves have
I spent a long time analyzing this situation especially in relation to how momentum is conserved and the kinds of forces exerted in the process. It is now on the arxiv https://arxiv.org/pdf/1406.5123.pdf until I have time to pursue publication. There is also a second chapter to this article I need to finish first. The short answer is that the light is not speeding up or slowing down but it is getting partially absorbed and transformed into energy and momentum in the medium. There is a standing wave component that is generated in the medium despite the fact the incoming and outgoing waves have none. This is also a source of stored energy. What you interpret as slowing down in the “ring up” energy and associated time lag as a packet moved into the medium. This is a different issue than any change in the phase velocity which we use to describe diffraction.
In the case of negative index materials this picture clearly shows the advancing microscopic phases of the waves while the macroscopic fronts seem to act in reverse. It is a very compelling picture of how conserved quantities are locally described and unambiguously stored and transported in the medium.
Photons have zero rest mass, and therefore have zero kinetic energy. That means they don’t lose energy when they slow down, and they don’t need to gain energy when they speed up.
If they lost energy when they slowed down, the energy would heat up the glass. The glass does not heat up when light passes through it—except for the energy it gains due to light that doesn’t pass through because the glass is not perfectly transparent.
Here’s a convenient explanation that’s probably wrong: the light makes something in the glass vibrate, so that the light slows down, and then when the light gets through
Photons have zero rest mass, and therefore have zero kinetic energy. That means they don’t lose energy when they slow down, and they don’t need to gain energy when they speed up.
If they lost energy when they slowed down, the energy would heat up the glass. The glass does not heat up when light passes through it—except for the energy it gains due to light that doesn’t pass through because the glass is not perfectly transparent.
Here’s a convenient explanation that’s probably wrong: the light makes something in the glass vibrate, so that the light slows down, and then when the light gets through to the other side the vibration kicks up its speed again as it leaves the glass. A better explanation might be that the light energy is repeatedly being absorbed and re-emitted by the atoms in the glass, and between atoms the photons always move at the speed of light.
Light has wave-particle duality. That means that whenever it’s hard to explain it as a particle, you should look at it as a wave, and vice versa. In this case it’s more sensible to look at it as a wave. Waves don’t lose or gain energy when they slow down or speed up as they pass from one medium to another.
If light slows down when changing mediums, such as air into glass, how does it gain energy to speed up again when it comes out the glass?
Whenever light is forced to travel through a medium, it slows, even if the medium is absolutely clear. These media are scattering the light’s photons; they are absorbed by energizing electrons and then re-emitted which delays the “forward” momentum of a light beam. An opaque medium can be a virtually complete barrier to traveling light waves, of course; although the light often manifests in the infrared, heating up the opaque “barrier”. What determines scattering is a medium’s refractive index. It is not the same as transluscence, but there is a relatively high correlation.
There are other ways
Whenever light is forced to travel through a medium, it slows, even if the medium is absolutely clear. These media are scattering the light’s photons; they are absorbed by energizing electrons and then re-emitted which delays the “forward” momentum of a light beam. An opaque medium can be a virtually complete barrier to traveling light waves, of course; although the light often manifests in the infrared, heating up the opaque “barrier”. What determines scattering is a medium’s refractive index. It is not the same as transluscence, but there is a relatively high correlation.
There are other ways to “slow” the “forward progress” of light, including masking it and causing it to spiral, passing it through a Bose-Einstein condensate or through photonic crystals or by significantly increasing the refractive index using lasers. In some experiments, light has been stopped completely. But, it can be released and returns to its normal “straight” forward path upon doing so.
The tendency of light to travel at c does not require energy to reach such speed; it is a fixed natural phenomenon that exists as a given universal physical aspect/attribute/property, like force strengths, particle weights, etc. Energy levels affect only wavelengths (and hence the “color” of light), never speed. When the resistance presented by a filtering medium is released/removed, there is an instantaneous increase to c in accordance with its universal property. (BTW, we don’t know why the speed of light is what it is. Popular conjecture holds that it is a function of the structure space-time which establishes a “speed limit”. However, this is not well documented.)
To answer it really short, looking at a “single photon” the speed of light in vacuum actually doesn't have much to do with the speed it passes through a medium. It “jumps” from atom to atom, getting absorbed and emitted (not really the “same” photon, but you can think of it like that). The speed in the medium depends on how long it hangs out in each atom and how many it hits, but in between them, it travels with the speed of light.
The energy of a photon is equal to its frequency times Planck's constant. Its frequency doesn't change as it passes from one medium to another, therefore neither does its energy.
Great question!
As you mentioned, photons do not have “mass”. So what does “mass” even mean? Why does light travel at this speed?
Before we begin answering those questions, there is a little correction to your question we should address. Light is not the only thing which can travel at this speed in a vacuum. Gluons are another massless particle which mediate the Strong Force interaction. They also travel at the speed of light.
By the end of this answer, you should have a good idea of:
- Why massless particles move in the vacuum at light speed;
- What mass and massless mean on a very basic level;
- Why thi
Great question!
As you mentioned, photons do not have “mass”. So what does “mass” even mean? Why does light travel at this speed?
Before we begin answering those questions, there is a little correction to your question we should address. Light is not the only thing which can travel at this speed in a vacuum. Gluons are another massless particle which mediate the Strong Force interaction. They also travel at the speed of light.
By the end of this answer, you should have a good idea of:
- Why massless particles move in the vacuum at light speed;
- What mass and massless mean on a very basic level;
- Why things with mass cannot travel at the speed of light;
- The basic underlying reason why things with mass undergo time dilation and length contraction when they are not at rest;
But we will need to lay down a few fundamentals along the way. Ready to begin?
Einstein’s Nobel Prize
Pop quiz question: Do you know what Einstein actually won the Nobel prize for? Was it E=mc²? Special Relativity? General Relativity? How about none of the above!!
It was actually for explaining something called the photoelectric effect.
We won’t discuss all of it, but we will discuss what it means for space and time.The explanation Einstein used had borrowed from Planck’s equation and it basically tells us that a measurement of light cannot be divided infinitely. Once you get to the smallest “piece” of light that can be extracted from the vacuum - a “photon” he called it - you cannot divide it any further. This opened up a new field of study called Quantum Mechanics. This is the study of the behavior of particles which cannot be further divided called “quanta”. That comes from the word we get “quantity” from which means you can count them!
The Planck Units
The idea that particles and light have a smallest component not only helped to explain many phenomenon, but it also gave physicists a way to apply this concept to space and time. But how can we go about finding the smallest length and time units if our measuring sticks are many orders larger? This is where physicists got creative and used a mathematical tool called “non-dimensionalization”. This led to the calculation of natural units of time, length and mass - called Planck units.
For example, Planck length relates to meters by:
[math]1 \ell_P=1.61622837*10^{-35} m[/math]
And Planck time relates to seconds by:
[math]1 t_P=5.3911613*10^{-44} s[/math]
So that means these units are extremely small. In fact, if you wanted to count how many Planck lengths fit in a meter, you would need around 61,872,444,424,420,000,000,000,000,000,000,000 of them!!
And I’m not even going to try to write out Planck time units per second. The point is these are very, very small units.
Speed of Light is Planck Velocity
The interesting part about Planck units and the speed of light is that if you then divide Planck length by Planck time, you will get exactly the speed of light in a vacuum. So
[math]1\ell_P/1t_P=299,792,458 m/s[/math]
Isn’t that cool?
So what does that even mean about the speed of light? This means that the fastest anything can travel is exactly 1 length unit per 1 time unit. I want you to think about the King in the game of chess. You can move the king any direction you want, so long as you only move one unit per turn(except when you castle! I just broke my own example…).
So it simply means everything cannot skip length units for each unit of time.
So what prevents things with mass from reaching light speed?
For this, would like to introduce you to a fun and simple little puzzle which I developed for teaching the “why” behind special relativity. You won’t need any math, and the puzzle is so simple that even a 5-year-old would be able to understand length contraction and time dilation from special relativity. If you’ve ever played checkers(or any board game) and if you are able to follow some simple rules, in 5 minutes you will understand the fundamentals of special relativity. You can find that here:
Special Relativity Using Two Dimensions and Planck Units
If you did the puzzle above, you will see that the cyclical nature of the waves which compose particles with mass, when these are traveling through Planck length and time units, this will naturally lead to length contraction and time dilation. You may also see that this cyclical nature of particles with mass is what gives them all of the properties of mass. It is as though their cyclical nature is the underlying property which makes them have mass - the two are interchangeable. Meanwhile, light and gluons - massless particles - proceed in straight paths(the z-axis center of their wave function) which allows them to constantly move at 1 Planck length per Planck time - which is the speed of light! Would it surprise you to learn that Planck mass(which we didn’t talk about, yet) has a Compton wavelength of 2π Planck lengths, exactly the circumference of a circle with a radius of 1?!
Smaller in the quantum realm equals more energy and mass. So a Planck mass particle is quite massive as far as particles are concerned. In fact, in the last century many theoretical physicists have often wondered why Planck mass is so massive! But the often overlooked fact is that it is the smallest in terms of radius and not in terms of mass.
Please put any questions or thoughts in the comments below.
If you found this answer helpful, please upvote so that others can find it. And thank you for reading.
Update: Broke 1,000 upvotes and looks like it’s still climbing. Thank you, everyone, for your votes.
I did want to address a point that arose in the comments. Planck length and time does not necessarily imply that space and time are pixelated.
Footnotes
A photon’s ‘slowing down’ when it passes through glass (or any matter, if it can do so without absorption) is simply its taking a nonlinear path ‘between’ (or ‘around’) the glass’ atoms… if it makes it through, it will ‘resume’ its former straight-c path.
This is a bit more difficult. Basically, the photons take a longer path through interaction with electrons in the material. A reasonably explicit account can be found in Feynman's book on quantum electrodynamics, which is quite approachable.
This is a difficult subject to make intuitive because it disagrees with our cultural and innate perceptions of time. But it makes sense if you look at it the right way, specifically by considering spacetime as one continuum that we move through instead of space and time being entirely separate dimensions.
The speed of light (SoL) is a limit to how fast things can move through space, but that's really only one side of the coin. There is also a "speed limit" for moving forward in time. By move forward in time, what I mean is the rate things change or age -- for example the rate of chemical reacti
This is a difficult subject to make intuitive because it disagrees with our cultural and innate perceptions of time. But it makes sense if you look at it the right way, specifically by considering spacetime as one continuum that we move through instead of space and time being entirely separate dimensions.
The speed of light (SoL) is a limit to how fast things can move through space, but that's really only one side of the coin. There is also a "speed limit" for moving forward in time. By move forward in time, what I mean is the rate things change or age -- for example the rate of chemical reactions, heat transfer, and nuclear decay. Those processes can't occur any faster than they occur in a "resting reference frame." We don't really have appropriate units to express this rate of change, but I'll call it 1 local-second per resting-frame-second. (I just made that up.) It means time is moving forward at the "normal" rate.
This "underlying rate of change" of 1 second per second is what it is, and if it were anything different, our clocks wouldn't be able to tell because they would speed up or slow down along with everything else. We can only tell how fast time is moving relative to other reference frames, for example a rocketship accelerating close to the speed of light. In that rocketship, time might be advancing at the slow pace of 0.1 local-second per resting-frame second. Relativity allows time to slow down, but it doesn't allow it to go faster than 1, whatever it is that "1" actually means.
So on one side of the coin, you have a speed limit (motion through space), and on the other side of the coin, you have an aging limit (motion through time). But speed and aging are both rates of change in spacetime, and they are linked in an extremely important way. To simplify the math (a lot), you could say that an object's "forward speed in time" plus its "forward speed in space" is a fixed number. Objects at rest have the fastest rate of aging, and objects travelling at the speed of light through space do not age at all. Objects somewhere in the middle are moving in both space and time, according to the equations of relativity. speed+aging=constant.
This means is that light does not experience time -- from the subjective viewpoint of the photon, it does travel infinitely fast, and arrives at its destination in the same instant it leaves its source. If you were a photon, you wouldn't have any way of determining or expressing what the speed of light is. You wouldn't perceive distance or the passage of time, so the idea of dividing distance by time simply wouldn't make any sense.
Likewise, an object at rest (or close to it, like us here on Earth) has great difficulty perceiving time in any absolute sense, because we don't have any way of measuring time that isn't itself advancing in time. We don't have any objective means of identifying what the "rate of time" actually is.
Circling back to the point -- there is a speed limit for motion through time, and a speed limit for motion through space, and they're actually just opposite extremes of the same thing. All objects move through spacetime at the same rate in all situations. There is only one fundamental rate in the universe, and nothing ever goes any slower or faster. Call it the "speed of speed" if you want. If you're not moving through space, you must be moving through time at the maximum possible rate. If you're not moving through time, you must be moving through space at the maximum possible rate.
So what the speed of light actually represents is the conversion factor between time-motion and space-motion. The exact number (299792458 m/s or whatever) is arbitrary because our choice of units (meters, seconds) is arbitrary. But we can conclude that 299792458 meters and 1 second are equal quantities of spacetime.
If we actually defined the units of distance and time in an appropriate way, the speed of light would really just be 1. It doesn't make mathematical sense for the speed of light to be faster than 1, just like it doesn't make sense for the rate of time to be faster than 1. That's just how fast the universe changes. It is what it is, and subjective observers inside the universe can't perceive it to be anything different.
Yes, light slows down a lot when travelling through some material, even through air.
It is this slow-down that refracts the light, too. The more refraction you see in a material, the more it slows light down.
In some weird materials, you can even make light move slower than you can run.
This is not relevant to the physics that include the constant “c”, though, as “c” is not actually the speed of lig
Yes, light slows down a lot when travelling through some material, even through air.
It is this slow-down that refracts the light, too. The more refraction you see in a material, the more it slows light down.
In some weird materials, you can even make light move slower than you can run.
This is not relevant to the physics that include the constant “c”, though, as “c” is not actually the speed of light, it is the speed of causality. What I mean is that it doesn’t matter that you can move faste...
What if we change the whole paradigm? That is, change the current academic model that claims that light “travels” through space over time (and according to Einstein, connects the past to the present instantaneously). — If there is truly instantaneous light communication via the special theory of relativity, then why the premise that light must “travel” over time? Why not postulate (3-D only) fields of electromagnetic energ
What if we change the whole paradigm? That is, change the current academic model that claims that light “travels” through space over time (and according to Einstein, connects the past to the present instantaneously). — If there is truly instantaneous light communication via the special theory of relativity, then why the premise that light must “travel” over time? Why not postulate (3-D only) fields of electromagnetic energy in real-time from irradiating sources.
The reason is because of how electromagnetic energy is measured. The light energy is measured by the number of oscillations it produces in a specific period of time by means of the electric and magnetic fields from which it emanates as per the field’s source. This we call electromagnetic radiation i.e. light. Because this energy is treated like classical waves through space over time with frequency, it can be measured as such. And for the most part, the math works. Look up phase, polarization, coherence, Maxwell’s equations, the Lorentz force law, among notable others, and you will see an astounding collection of some very functional mathematics to describe the phenomenon.
So yes, the math works, just like Einstein’s and Newton’s theories both “work” but is the premise and understanding of light correct?
What would the paradigm shift be? TIME.
EM radiation as an instantaneous Newtonian field similar to gravity. This would explain electromagnetic “speed” changes through a medium in a new light. That is, the electron matrices of translucent objects interact with an EM-field in different ways as types of partial interference. Therefore, glass and water do not “slow” light down, but rather affect the “path” or manifestation of the “wave” (field energy) in such a way as to change the directional flux of the radiation due to the interference; thus, light can be focused etc. — A “photon” then, might be better defined as a specialized vector equivalence; the minimum quantum of specific energy displacement from an EM-field.
Mathematically the EM-field can still be described and measured as “photon-waves” traveling over time, but the reality I submit has nothing to do with time at all, other than the present moment of “now” in which the radiation is manifest and interacted with. This would make light (live EM-field energy) unique compared with other forms of “wave” energy, which I believe it is.
Anyhow, at this point it is still partially conjecture, thanks for reading.
Two ways to think about this:
- Light is photons. The energy of each photon is given by its frequency; it doesn’t depend on its speed. Since the frequency (color) is the same in the glass, there’s no energy to gain or loose: It’s been the same all along.
- Even as a classical wave, energy isn’t “lost” while in the glass. A tiny fraction of the energy is delayed by interactions with the atoms (really the electrons) in the glass. That’s why the light is slowed down, little parts are delayed, and the further it goes through the glass the more that happens. so it acts like it’s being delayed by having a
Two ways to think about this:
- Light is photons. The energy of each photon is given by its frequency; it doesn’t depend on its speed. Since the frequency (color) is the same in the glass, there’s no energy to gain or loose: It’s been the same all along.
- Even as a classical wave, energy isn’t “lost” while in the glass. A tiny fraction of the energy is delayed by interactions with the atoms (really the electrons) in the glass. That’s why the light is slowed down, little parts are delayed, and the further it goes through the glass the more that happens. so it acts like it’s being delayed by having a lower speed. But no energy is being lost due to this and therefore nothing needs to be regained. Once it leaves the glass, it’s not interacting any more, nothing is being delayed, and it’s back to it’s prior speed.
Because photons have no rest mass they must travel at the speed of light. In a vacuum or through a refractive media such as glass the rule applies. The speed of light in glass is slower than it is a vacuum. The relationship between the speed of light in a vacuum and that in a refractive medium is called “refractive index”.
Index of refraction = speed of light in vacuum/speed of light in refractive medium.
Light does not increase in speed after it leaves a medium.
Light that does not go through a medium travels direct from point A to point B.
Light passing through a medium hits particle 1 in the medium then bounces off at an angle hitting point 2 then goes off at another angle hitting another point etc. etc. until it emerges at point B.
It does not go slower it goes farther and it takes longer to go farther. traveling at the same speed.
Technically they do not bounce off atoms they are absorbed and then emitted when they hit atoms.
In effect, yes. The idea that light is slowed down by traveling through glass is really a misrepresentation of what's happening. A given photon doesn't really slow down. It always travels at the speed of light, which is fixed.
What happens when light passes through glass is that a photon hits an atom and disappears, driving one of the electrons on the atom into a higher energy state. A fraction of a second later, the electron drops back down to its original lower energy state, and the atom emits a new photon. This process is what actually takes the time. Both photons move at the speed of light,
In effect, yes. The idea that light is slowed down by traveling through glass is really a misrepresentation of what's happening. A given photon doesn't really slow down. It always travels at the speed of light, which is fixed.
What happens when light passes through glass is that a photon hits an atom and disappears, driving one of the electrons on the atom into a higher energy state. A fraction of a second later, the electron drops back down to its original lower energy state, and the atom emits a new photon. This process is what actually takes the time. Both photons move at the speed of light, but that little fraction of time between the disappearance of the first one and the appearance of the new one is what creates the impression that the light has “slowed down.”
Outside the glass, the photon continues at the speed of light, giving the impression that it “sped up again.”
The energy of photons depends on their frequency, not their velocity. The frequency is unaffected by the medium that they move in.
Each photon under those conditions has a constant energy at all times, in or out of the glass. However, the photon loses momentum to the glass as it enters the glass. The photon gains momentum from the glass when it leaves the glass.
The light is made of many photons, not just one. The light beam will lose photon to reflection at both surfaces. Hence, the transmitted light has contributed energy to the reflected light.
The easiest way to get the exact behavior is from thinking about light as a classical wave interacting with the atoms in the material medium.
one can think each of the atoms as being like a little dipole, that is driven back and forth by the off-resonant light field.
Being an assemblage of charges that are accelerating due to the driving field, these dipoles will radiate, producing waves at the same frequency as the driving field, but slightly out of phase with it (because a dipole being driven at a frequency other than its resonance frequency will be slightly out of phase with the driving field
The easiest way to get the exact behavior is from thinking about light as a classical wave interacting with the atoms in the material medium.
one can think each of the atoms as being like a little dipole, that is driven back and forth by the off-resonant light field.
Being an assemblage of charges that are accelerating due to the driving field, these dipoles will radiate, producing waves at the same frequency as the driving field, but slightly out of phase with it (because a dipole being driven at a frequency other than its resonance frequency will be slightly out of phase with the driving field).
The total light field in the material will be the sum of the driving light field and the field produced by the oscillating dipoles.
This delay registers as a slowing of the speed of the wave passing through the medium.
The basic idea of treating the atoms like little dipoles is a variant of "Huygens's Principle," by the way, which is a general technique for thinking about how waves behave.
When it comes out its the wave with its faster speed.
One does not need a push or impulse to the wave to regain its velocity.
ref.-What is the mechanism behind the slowdown of light/photons in a transparent medium?
Light doesn't lose energy when entering glass from vacuum nor gain energy when leaving glass into vacuum. In glass the properties of glass change the magnitudes 8f e and u so that light wavelength shortens, though the frequency of the light, determined at its source, remains the same. So energy E = hf is the same in both media. The difference is that in glass the value of c, the intrinsic speed light must have to exist, is slower than the value of c fixed in vacuum. Sunce c=fl and f is the same while l is smaller, c is also smaller. So E = hf = hc/l produce the same computed energy for each ph
Light doesn't lose energy when entering glass from vacuum nor gain energy when leaving glass into vacuum. In glass the properties of glass change the magnitudes 8f e and u so that light wavelength shortens, though the frequency of the light, determined at its source, remains the same. So energy E = hf is the same in both media. The difference is that in glass the value of c, the intrinsic speed light must have to exist, is slower than the value of c fixed in vacuum. Sunce c=fl and f is the same while l is smaller, c is also smaller. So E = hf = hc/l produce the same computed energy for each photon traveling through the various media
You have the concept of speed of light inverted. You believe traction is necessary for speed. However, traction is necessary for acceleration of mass. And light passing through a pane of glass doesn’t accelerate, it changes reference frame. The light always has the same speed (“causally instantaneous”) and it’s only our measurement and attempt to translate that to a physical reference frame that changes.
In short, the energy a photon carries solely depends on the frequency as given by E=hv , where v is frequency and h is planck constant. When light travels from one medium to another, it speed and wavelength change but frequency doesn't. Hence no extra energy is gained.
No brother, like the other comments have said, that's not how it works.
See, a photon's whole identity is wrapped up in moving at the speed of light. It's like saying a bullet keeps its 'bulletness' after it's embedded in a wall.
When a photon gets absorbed by an electron, it's game over for that photon.
Ceases to exist, see?
Its energy gets transferred to the electron, bumping it up to a higher energy level, like a caffeine hit for an atom.
Now, this electron might later chill out and drop back down to its original energy state, spitting out a new photon in the process.
But that new photon is a who
No brother, like the other comments have said, that's not how it works.
See, a photon's whole identity is wrapped up in moving at the speed of light. It's like saying a bullet keeps its 'bulletness' after it's embedded in a wall.
When a photon gets absorbed by an electron, it's game over for that photon.
Ceases to exist, see?
Its energy gets transferred to the electron, bumping it up to a higher energy level, like a caffeine hit for an atom.
Now, this electron might later chill out and drop back down to its original energy state, spitting out a new photon in the process.
But that new photon is a whole different entity, not the original one resurrected.
Kinda like them relay races.
The first runner hands off the baton, then collapses on the track. The second runner takes off with the baton, but it's a new leg of the race.
So, the original photon's speed of light journey ends abruptly upon absorption. It's a one-way ticket, no refunds.