Question about light

I have no back-story for this Q, it is what it is-that said.

Can light bounce off/reflect/be redirected by other light?
This is of course asking this through no other means of reflection, say a mirrored surface or what not.

Light can set up interference patterns, illustrating its wave nature, but particles of light don’t bounce off each other like billiard balls. If you shine two flashlights in the same plane at right angles, the beams of light will not deflect each other. You can’t shine a light into the air and then show a movie on that light.

I suspect that a real physicist will stop by and tell you much more.

When light “bouces off” materials, what is happening is that it is absorbed by an atom of that material, and then re-emitted. The atom literally absorbs the energy of the photon, and you can think of it as no longer existing-- it’s energy is transfered to the atom, putting the atom in a higher energy state than it previously was. The photon emitted from the atom is a “new” photon created from the atom returing to a lower energy state. But the photons themselves can’t absorb energy like an atom can, so there is not mechanism for it to “bouce off” another photon.

As a further example, we often talk of light having a slower speed in a medium. But it’s not that the light is literally travelling more slowly, it’s just being absorbed and re-emitted. The only travelling it actulaly does is thru the space between that exists in the medium iteslf. Atoms are mostly empty space, remember, so most of what we think of as a solid or a liquid is simply space.

So how does the atom “know” to follow that angle rule I can’t remember the name of. For example, if I shine a flashlight at a mirror so that the light is moving upward at a diagonal of 10 degrees below the horizontal, it will be reflected at 10 degrees above the horizontal. How does the atom “know” which way the light came from so that it will also “know” which direction to expel the new photon?

It’s very rare but it is possible to have photon photon scattering. The photons have to have very high energy and it involves a: photon --> positron electron pair --> scatter --> photon: type of interaction.

Here’s a quote from one of my favorite physicists, Matti Meron:

Ring, I think we need to ask Mr. Meron why:

If the re-radiation is in all angles, and only one can be seen (all others are destroyed by interference?), why does that one have almost all of the energy of the original? After all, good mirrors reflect about 98% of what they receive.

You answered your own question: because all the others are destroyed by interference. The energy of a light wave is proportional to the square of its amplitude; if there’s very very little amplitude to the wave in a given direction due to interference, then there’s even less energy.

This isn’t pure photon-photon scattering . A photon can’t just turn into an electron/positron pair in thin air*; it needs to hit a massive particle, such as an electron.

*Of course if it’s literally thin air, that would provide enough air molecules for this interaction. What I really mean is perfect vacuum.

When waves destructively interfere it’s just like they were never emitted.

Actually, it can, as long as it doesn’t stay that way for long. The electron and positron are virtual, so they’re allowed to break the rules. And in principle, the process Ring described can actually occur for any energy of light; it’s just far less likely at low energies.

Another quote, this time from the eminent John Baez:

Hmm, OK - could you remind me what this process is called?

However the eminent Chronos’ explanation was even better.

That sounds suspiciously like a violation of conservation of energy.

Quite the contrary in fact; the system as a whole requires it - otherwise the constructive interference elsewhere in the system would be pulling energy out of nowhere.