Materials are not ‘solid’. They’re made up of atoms, and a photon is still smaller. How is a photon reflected by material that it interacts with?
It’s all about the electrons…
Wikipedia has a basic explanation here.
Essentially, an incident photon causes electrons in the material to oscillate, and they re-radiate energy which ends up being the reflected light.
They are absorbed by the material, exciting the electron(s) into a higher energy state. That state is often not stable, and when the electron decays back to the original state, a photon is emitted.
If it’s emitting a new photon, why does that photon exit opposite to the angle of incidence? I can understand solid objects acting like this since the surface they strike bounces them off. But if a photon excites an electron and then the electron emits a different photon, why isn’t the new photon going at a completely random angle?
Photons are not so small- it depends on their wavelengths, but visible ones usually can not penetrate matter (unless it is transparent, but that’s another story). Photons can only reliably pass through gaps that are smaller than their respective wavelengths.
This is why, for instance, in a microwave oven, you can see large holes in the metal mesh- the microwaves have a larger wavelength than the holes, and so do not escape through them.
That isn’t always the case; a beam of light will reflect off a mirror in the opposite direction but if you shine a light on a wall (just an ordinary white wall) the light will reflect off in all directions. These are examples of specular and diffuse reflection respectively. The difference basically is that in diffuse reflection light is reflected from beneath the surface, even in otherwise opaque materials (the same angle of incidence/reflection principle still applies; the light just reflects off of numerous boundaries). Of course, this then doesn’t really answer the question of why light reflects in the opposite direction; the answer appears to be that light is also like a wave and waves reflect in such a manner.
Is a photon “small”? When a single photon passes through a wide slit, where it falls on a screen afterwards ends up contributing to the characteristic diffraction pattern of the slit. If you stop down the intensity so that you can be sure only one photon is passing through at a time, after a long time you find that you have your diffraction pattern. Clearly the photon has detected the width of the slit in order to fall into the probability distribution that leads to that pattern. This means that the photon “reaches” out to a distance vastly larger than its expected size. At the same time, a photon can pass through a tiny hole just larger than the wavelength of the light. Is the photon tiny 9to pass through the holes) or large (to sense a wide slit)? The answer has to be both. I sort of mentally imaging a photon as a fuzzball of energy that is mostly compact, but with long “tails” that go pretty far out. It’s probably not a correct picture, but it does give you a mental picture that goes with what happens.
As for how a photon bounces, you can look at it from a particle or and electromagnetic wave point of view. Electromagnetic waves fulfill boundary copnditions and produce a reflected wave. Have a look at any book of electromagnetic theiry – Stone or Jackson or Marion or any of the newer ones. On a particle level, as noted, the photons get absorbed and re-emitted, but the energy and momentum they carry stuill gets conserved.
Yeah, that conservation of energy and momentum is sometimes the very thing that gets a lot of people confused about how it all works.
We all live in a ‘Newtonian’ world. This colors our perceptions on a fundamental level.
Entities like particles and atoms act in a qualitatively different manner than the objects that they comprise.
Some folks (not the folks around here… I mean… ummm… those other folks over there) never really grasp this. Atoms aren’t like the things that they make up. Atoms don’t have colors, for example.
Here’s a closely related question. The answer to this and to the OP are either identical or can at least shed a lot of light on each other. Namely:
How is it that a lens [i.e., any transparent substance] will affect the direction that the photons are traveling?
So it is or is not the same photon that ‘bounces’?
It’s not. The photon is absorbed and a new one is emitted when the relevant atom’s electrons collapse back to their original states. Don’t think about this in terms of Newtonian physics, because it won’t make sense. Remember that a photon is both a particle and a wave: the discreet particle is absorbed and reemitted, but the wave is reflected.
That’s because the white surface of the wall is composed of myriad, tiny, randomly-oriented surfaces. Equality of angle of incidence and reflection is still at play, so this doesn’t answer the question.
I know pointing to a wiki article is a bit of a cop out, but the next best alternative was an Amazon link, and I didn’t want to do that.
Anyhow, the answers are to do with how photons move from place to place and interact with electrons (for starters, photons don’t “bounce”). It’s a bit complicated but it’s all in the Richard Feynman book…
QED: The Strange Theory of Light and Matter.
Reflections are in lecture 2. Happy reading.
One of the things I was thinking of was experiments where they said they bounced photons back and forth. As I understood it, it wasn’t the same photon being bounced, as kombatminipig says. Which would seem to corrupt the idea of “we bounced a photon back and forth”. No, it wasn’t the same photon.
It might be a moot point, depending on whether a photon is a ‘thing’ or a phenomenon.
If I toss a pebble in a pond, it makes ripples - when those ripples reach the edge, are they the same ripples as they were at the start? Not arguing for the existence of Luminiferous Aether or anything, but I’m not sure whether ‘the same’ and ‘different’ are as useful terms here as they would be if we were talking about, say, tennis balls instead of photons.
For things like particles, saying that two instances are “the same” or “different” particles loses all meaning. After all, you can’t paint a smiley face on a photon and check it again for the smiley face to see if it is the same one.
And this has effects on the statistical properties of the interactions. For example, you won’t get the right probability amplitudes for the scattering of a free electron off an atom unless you also account for the possibility that the free electron gets absorbed by the atom at one of the atom’s electrons is emitted in its place, since there’s no way to tell that that didn’t happen.