Main Question or Discussion Point
I did a search for optical accelerator but it's not clear to me exactly what is being discussed. If so, has the been an invention to do this?
In that case (air to water) it would decelerate, not accelerate since water is denser than air. As I just found out photons have a maximum speed. Think of light traveling from the sun to earth. It takes around 8 minutes for it to get here even though it is moving through a vacuum, it has a maximum speed: 299,792,458 m/sWhat if a photon is traveling from one medium to another, both with different indices of refractions (like from air to water)? The velocity of the photon would change upon entering the second medium, would that not be considered an acceleration? Or am I missing something..
Perhaps the following post in the "Physics FAQ" which is stickied at the top of this forum, will help:What if a photon is traveling from one medium to another, both with different indices of refractions (like from air to water)? The velocity of the photon would change upon entering the second medium, would that not be considered an acceleration? Or am I missing something..
Is this true? I thought photons are absorbed and new photons re-emitted, from a mirror.While the speed of light is constant, the velocity of a given photon most certainly isn't. Mirrors for example technically act as photon accelerators by changing the direction of propagation of photons.
It has been my understanding that photons (I don't like that term after Aspect) always travel at the speed of light, and that the reason light signals travel more slowly inside of materials is because of an absorption-emission processes.While the speed of light is constant, the velocity of a given photon most certainly isn't.
Yes indeed. A photon's interaction with matter can be described in terms of absorption and emission.Is this true? I thought photons are absorbed and new photons re-emitted, from a mirror.
I think that some confusion about "the speed of photons" comes about because we forget how photons are defined, and start thinking of it as little "light bullets", or "wave packets" of classical EM radiation.Is this true? I thought photons are absorbed and new photons re-emitted, from a mirror.
Ok. But I haven't completely understood what happens.I think that some confusion about "the speed of photons" comes about because we forget how photons are defined, and start thinking of it as little "light bullets", or "wave packets" of classical EM radiation.
In free space, photons are defined as the different eigenstates of the energy and momentum operators of the EM field. It turns out that they correspond to harmonic oscillator solutions for each and every mode of the classical EM field, which, in free space, comes down to plane harmonic waves. So to every classical plane harmonic wave corresponds a quantum-mechanical harmonic oscillator (you know, with energy levels E_n = (n + 1/2) hbar omega), and we call the steps between these energy levels: photons.
If there is a dielectric, we can treat the situation in two different ways. We can have a kind of semiclassical approach, where we look first at the *classical* modes of this setup, and then assign again quantum mechanical oscillators to each of these classical modes, and call them "photons". Or we can treat this dielectric as a quantum-mechanical system which *couples* to the free EM field.
In the first case, you understand that the "photons" of this semiclassical system are ENTIRELY DIFFERENT states than the photons of the free EM field. They are eigenstates of a totally different problem, and hence the "free EM photons" have not much to do with the "semiclassical dielectric photons".
However, because the dielectric interaction has been taken into account (classically), there is no "interaction of these photons with the dielectric": it is already included in the classical field solutions that were to be quantized.
In the second case, the interaction terms between the free EM field and the quantum mechanical dielectric system, which induces transitions between the "free photon states", because of the couplings (perturbations) introduced by the dielectric (seen as a quantum system that introduces interaction terms in the EM hamiltonian).
You mean, with the mirror ?Ok. But I haven't completely understood what happens.
Let's say a single photon is involved in the process. How could you explain it in simple terms (if it's possible)?
It really depends on your description. I think you are referring to incoherent absorption and re-emission. That's true, it is not an incoherent absorption and re-emission. However, the process can be modeled by a destruction operator of the incoming beam and a creation operator in the outgoing beam, when you consider the beams as those of the free EM field (without mirror).Reflection is not an absorption/emission process, nor is transmission through a transparent medium.
Light in a vacuum is the one or stream of photons of light, they travel until they strike some mass, they are absorbed into that mass. If that mass is made of certain stuff, when the light is absorbed, at almost the same time another photon of light is emitted.
How does the new photon "know" in which direction to go in order to be properly reflected (or, sent) in an optical instument?