Light Question

[fcunnane]

If the speed of light in a "vacuum" is considered to be a constant, and light traveling through any medium is logically reduced in speed, theoretically, if the light traverses the medium to a vacuum will it accelerate to c and finally reach c once total vacuum is realized?

Logical Experiment:

Light Source is located within a medium.
Light Source generates light.
Light travels away from source through medium.
Light presumably traverses medium.
Light enters a vacuum.

Will the light accelerate on the way to realizing a true vacuum without any external transfer of energy to create this acceleration?

[caveman1917]

One mechanism that i'm aware of that makes the speed of light 'reduce' in a medium, is the fact that the photons get absorbed (excite) the atoms, then after a while the atom de-excites and emits the photon again. The reduction in travel speed is thus an effect of the 'waiting time' for the atoms to de-excite. Between the atoms the photons still propagate at c.

[Ken G]

And yes, the energy of the photon is determined by its frequency, not its speed, and frequency stays the same in the medium and the vacuum, so the energy stays the same. Interestlingly, there remains some dispute over the correct interpretation of the momentum of a photon in a medium, though some light may be shed on that debate at http://physicsworld.com/cws/article/news/41873.

[fcunnane]

I was under the impression that the light had to be at an exact wavelength to excite the atom. Otherwise the atom(system) is not affected by the wavelength and it passes.

Something like: Atoms can make transitions between orbits by emitting and absorbing exactly the energy difference between the orbits.

After absorption, would there be a requirement of outside energy for the atom to return to it's original orbit to release the energy difference in orbits following E = hc / λ?

[Ken G]

I was under the impression that the light had to be at an exact wavelength to excite the atom. Otherwise the atom(system) is not affected by the wavelength and it passes.

That is only true at low density. At high density, the atoms are perturbed by their environments, and we get continuous emitters like "incandescent light bulbs." At high enough density, you can treat the medium as a single substance, with an "index of refraction" that slows the velocity of the wave.

After absorption, would there be a requirement of outside energy for the atom to return to it's original orbit to release the energy difference in orbits following E = hc / λ?

Energy has to be conserved.

[fcunnane]

That's cool...

Now what about the average density of space being very very low per cubic meter as far as atoms(systems) to absorb and emit energy, I would describe space here as a soupy medium of charged particles everywhere and not a vacuum... Could you imagine that light leaving the gravity wells of the earth would accelerate due to the laser on the surface being in a molecule medium, traversing the atmosphere and entering a less dense medium?

Edit: And yes the debate makes sense, either between wave or particle calculations. Books must balance I guess....

[fcunnane]

Does light act more like a particle in relation to mass and more like a wave in relation to space(lack of mass)?

[Ken G]

Could you imagine that light leaving the gravity wells of the earth would accelerate due to the laser on the surface being in a molecule medium, traversing the atmosphere and entering a less dense medium?

Yes, you could think of it that way. Again we have a different way of looking at it from the particle or wave perspectives. From the wave perspective, we have refraction, and the light wave does indeed "accelerate" as it leaves the atmosphere. From the particle picture, as we heard above, the photon propagates between interactions through the vacuum between the atoms at c, and it is the interactions that impede its progress. In the latter picture, we wouldn't say the light "accelerates", only that it makes more rapid net progress.

[Ken G]

Does light act more like a particle in relation to mass and more like a wave in relation to space(lack of mass)?

You are asking a very important and tricky question about light, which is, when does it act like a particle, and when can we think of it as a wave? One way to slice that question is to think in terms of distance scales. To treat light as a particle, it needs to have a position, and there will always be some uncertainty in that position. But the rule of thumb is, whenever the uncertainty in position is small enough to "not matter", the light may be treated as a particle, and when that is not true, it must be treated as a wave.

The classic example of when to treat it as a particle is when you do a position measurement, for then you know its position to within the uncertainty in your measurement, and by definition, that makes the uncertainty small enough for it to not matter to the physics. The classic example of when to treat it as a wave is when you do a momentum measurement, and then the "de Broglie wavelength" is a concept that arises from the ratio of h to the momentum you measure. A wavelength is a wave notion, and it means that the position of the particle is uncertain to many de Broglie wavelegths, depending on how precise is the momentum measurement. That makes the uncertainty in the position matter to the physics, so you have a wave situation.

More generally, you will have some distance scale of importance, like the average distance the particle travels before interacting. That becomes the "distance that matters", and if the location of the photon is actualized in the reality to well within that distance, you may treat the photon as a particle, and not worry about refraction. If, however, the location of the photon is not actualized to within that length, you must treat the photon as propagating like a wave, and refraction will matter. You mentioned a laser beam, and a laser beam has a straightforward concept of how well the location of the photon is actualized by the laser-- it is called the "coherence length" of the laser (basically how many times the photon bounces back and forth down the laser tube times the length of that tube). So you must treat the laser like it is spitting out waves whenever the coherence length is longer than the distance the photon will go before interacting in the medium, but if the coherence length is much shorter than that, you can imagine the laser is shooting out little bullets that always travel at c until they hit something.

[fcunnane]

Wouldn't the "bending" of light around let's say as the observer views an eclipse of that light source if the eclipsing object is large enough also be a symptom of acting as a particle? The gravitational force rather than the electromagnetic force of the wave function?

I guess what my real question here is, would it be logical to calculate the exact nature of light as a proportion that includes a ratio calculation of wave and particle interpretations. Is this what is currently being used to determine if the uncertainty in position is small enough not to matter?

[Ken G]

Wouldn't the "bending" of light around let's say as the observer views an eclipse of that light source if the eclipsing object is large enough also be a symptom of acting as a particle? The gravitational force rather than the electromagnetic force of the wave function?

Gravitational bending doesn't adjudicate between wave and particle, it's there for both-- it's just an effect on the background spacetime through which the particle or wave is moving. The length scale comparison is what adjudicates between wave and particle behavior.

I guess what my real question here is, would it be logical to calculate the exact nature of light as a proportion that includes a ratio calculation of wave and particle interpretations. Is this what is currently being used to determine if the uncertainty in position is small enough not to matter?

It's more a question of what is the size scale in the environment that the light encounters. For refraction, the coherence length has to be longer than the mean free path. For diffraction, the wavelength has to be longer than the size of the slit or object.

[fcunnane]

Gravitational bending doesn't adjudicate between wave and particle, it's there for both-- it's just an effect on the background spacetime through which the particle or wave is moving.

Is there something or some process or force that you can think of that does adjudicate between wave and particle as far as light is concerned?

Edit: Would you say that gravity and the electromagnetic forces are the adjudicates themselves...

[Ken G]

Is there something or some process or force that you can think of that does adjudicate between wave and particle as far as light is concerned?

Edit: Would you say that gravity and the electromagnetic forces are the adjudicates themselves...

I would say that the forces that are relevant to light, such as gravity and electromagnetic forces, can operate in a wave treatment or a particle treatment-- they are in a sense orthogonal issues to the wave/particle duality. To me the difference between waves and particles are not about nature by itself, they are about the questions we want to answer about nature, and our chosen scientific means for getting those answers, and that holds regardless of what forces are holding sway.

If I were to picture "nature itself", I would not picture something with both wave and particle aspects, I would picture something with neither. It just turns out that when we want answers to questions that relate to the location of the light (particle) to better than the precision of some distance scale of interest, we are asking nature a "particle question", and when we want answers to questions that require knowing the wavelength to high precision, we are asking nature a "wave question." I don't think nature asks itself these kinds of questions at all, so when queried in that way, its answer might be pictured "well I don't see why you want to know that, but since you do, I'm going to show you what you will think of as my wave nature, or my particle nature. For my own part, I just do whatever is natural, and you can picture it however you like, and scratch your head over the perfectly natural stuff that is nevertheless outside your everyday experience."

[HenrikOlsen]

Wouldn't the "bending" of light around let's say as the observer views an eclipse of that light source if the eclipsing object is large enough also be a symptom of acting as a particle? The gravitational force rather than the electromagnetic force of the wave function?

It's the space it's moving it which is bent, not the light.

As the light "experiences" things, it's moving in a straight line. It doesn't matter if it's wave or particle, it's just doing what light does when it's moving in a straight line.