# Question about light **URL:** \ **Category:** Factual Questions\ **Created:** [June 30, 2006, 1:25am UTC](https://boards.straightdope.com/t/question-about-light/362702 "2006-06-30T01:25:37Z")\ **Posts on this page:** 16\ **Page:** 1
**Author:** ![Thinktank](https://avatars.discourse-cdn.com/v4/letter/t/6a8cbe/32.png) [@Thinktank](https://boards.straightdope.com/u/Thinktank)\ **Post date:** [June 30, 2006, 1:25am UTC](https://boards.straightdope.com/t/question-about-light/362702/1 "2006-06-30T01:25:37Z")
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. ---
**Author:** ![CookingWithGas](https://sea3.discourse-cdn.com/straightdope/user_avatar/boards.straightdope.com/cookingwithgas/32/485_2.png) [@CookingWithGas](https://boards.straightdope.com/u/CookingWithGas)\ **Post date:** [June 30, 2006, 1:31am UTC](https://boards.straightdope.com/t/question-about-light/362702/2 "2006-06-30T01:31:31Z")
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. ---
**Author:** ![John\_Mace](https://sea3.discourse-cdn.com/straightdope/user_avatar/boards.straightdope.com/john_mace/32/185_2.png) [@John\_Mace](https://boards.straightdope.com/u/John_Mace)\ **Post date:** [June 30, 2006, 1:58am UTC](https://boards.straightdope.com/t/question-about-light/362702/3 "2006-06-30T01:58:55Z")
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. ---
**Author:** ![TJdude825](https://avatars.discourse-cdn.com/v4/letter/t/dec6dc/32.png) [@TJdude825](https://boards.straightdope.com/u/TJdude825)\ **Post date:** [June 30, 2006, 2:48am UTC](https://boards.straightdope.com/t/question-about-light/362702/4 "2006-06-30T02:48:21Z")
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? ---
**Author:** ![Ring](https://avatars.discourse-cdn.com/v4/letter/r/6a8cbe/32.png) [@Ring](https://boards.straightdope.com/u/Ring)\ **Post date:** [June 30, 2006, 3:23am UTC](https://boards.straightdope.com/t/question-about-light/362702/5 "2006-06-30T03:23:40Z")
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. ---
**Author:** ![Ring](https://avatars.discourse-cdn.com/v4/letter/r/6a8cbe/32.png) [@Ring](https://boards.straightdope.com/u/Ring)\ **Post date:** [June 30, 2006, 3:28am UTC](https://boards.straightdope.com/t/question-about-light/362702/6 "2006-06-30T03:28:50Z")
> [@TJdude825](#): > > 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? Here’s a quote from one of my favorite physicists, Matti Meron: > [@](#): > > No, it is not that obvious, though it is taken for granted by most > people. To see where it is coming from, you should think in terms of > waves. Imagine a plane wave impinging on surface and each surface > element, in turn, radiating in all directions. Now, the amplitude in > any direction is obtained by adding all the rereadiated wavelets which > ar going in this direction and, when you go through the details (not > very complicated but I’m not about to put it in ASCII) you find out > that in any direction you get waves of all phases, which cancel one > another. The sole exception is the specular reflection angle (i.e. an > angle equal to the incoming angle)\> Here everything adds in phase and > you get your reflection. ---
**Author:** ![K364](https://sea3.discourse-cdn.com/straightdope/user_avatar/boards.straightdope.com/k364/32/5_2.png) [@K364](https://boards.straightdope.com/u/K364)\ **Post date:** [June 30, 2006, 3:54am UTC](https://boards.straightdope.com/t/question-about-light/362702/7 "2006-06-30T03:54:19Z")
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. ---
**Author:** ![MikeS](https://avatars.discourse-cdn.com/v4/letter/m/919ad9/32.png) [@MikeS](https://boards.straightdope.com/u/MikeS)\ **Post date:** [June 30, 2006, 4:05am UTC](https://boards.straightdope.com/t/question-about-light/362702/8 "2006-06-30T04:05:30Z")
> [@K364](#): > > 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? 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. ---
**Author:** ![scr4](https://avatars.discourse-cdn.com/v4/letter/s/59ef9b/32.png) [@scr4](https://boards.straightdope.com/u/scr4)\ **Post date:** [June 30, 2006, 4:06am UTC](https://boards.straightdope.com/t/question-about-light/362702/9 "2006-06-30T04:06:47Z")
> [@Ring](#): > > 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. 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. ---
**Author:** ![Ring](https://avatars.discourse-cdn.com/v4/letter/r/6a8cbe/32.png) [@Ring](https://boards.straightdope.com/u/Ring)\ **Post date:** [June 30, 2006, 4:10am UTC](https://boards.straightdope.com/t/question-about-light/362702/10 "2006-06-30T04:10:28Z")
> [@K364](#): > > 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. When waves destructively interfere it’s just like they were never emitted. ---
**Author:** ![Chronos](https://sea3.discourse-cdn.com/straightdope/user_avatar/boards.straightdope.com/chronos/32/134_2.png) [@Chronos](https://boards.straightdope.com/u/Chronos)\ **Post date:** [June 30, 2006, 4:18am UTC](https://boards.straightdope.com/t/question-about-light/362702/11 "2006-06-30T04:18:20Z")
> [@](#): > > 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. 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. ---
**Author:** ![Ring](https://avatars.discourse-cdn.com/v4/letter/r/6a8cbe/32.png) [@Ring](https://boards.straightdope.com/u/Ring)\ **Post date:** [June 30, 2006, 4:20am UTC](https://boards.straightdope.com/t/question-about-light/362702/12 "2006-06-30T04:20:18Z")
> [@scr4](#): > > 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. Another quote, this time from the eminent John Baez: > [@](#): > > \>The idea that all charged particles are somehow > \>“connected” to each other and that the electromagnetic field is > \>just a disturbance of these connections seems to forbid such > \>photon photon scattering when no matter is present. > > I don’t really understand this remark, which is a bit vague > and seems to contradict your previous sentence. Anyway, there > doesn’t need to be any actual matter around; _virtual_ particles > are sufficient, and virtual particles are everywhere. > > \>And to date > \>I don’t think any photons have been observed to directly scatter > \>off of other photons (the interaction “far” from other charged > \>matter). > > The above experiment is, I believe, the recent famous experiment > in which photon-photon scattering was observed. This is as good > as it gets. You can argue the semantics of what “far” from other > charged matter means, but the point is, we have a Feynman diagram > with only photons coming in, and only photons with different momenta > coming out. ---
**Author:** ![scr4](https://avatars.discourse-cdn.com/v4/letter/s/59ef9b/32.png) [@scr4](https://boards.straightdope.com/u/scr4)\ **Post date:** [June 30, 2006, 4:23am UTC](https://boards.straightdope.com/t/question-about-light/362702/13 "2006-06-30T04:23:32Z")
> [@Chronos](#): > > Actually, it can, as long as it doesn’t stay that way for long. Hmm, OK - could you remind me what this process is called? ---
**Author:** ![Ring](https://avatars.discourse-cdn.com/v4/letter/r/6a8cbe/32.png) [@Ring](https://boards.straightdope.com/u/Ring)\ **Post date:** [June 30, 2006, 4:41am UTC](https://boards.straightdope.com/t/question-about-light/362702/14 "2006-06-30T04:41:34Z")
> [@Ring](#): > > Another quote, this time from the eminent John Baez: However the eminent **Chronos’** explanation was even better. ---
**Author:** ![TJdude825](https://avatars.discourse-cdn.com/v4/letter/t/dec6dc/32.png) [@TJdude825](https://boards.straightdope.com/u/TJdude825)\ **Post date:** [June 30, 2006, 5:40am UTC](https://boards.straightdope.com/t/question-about-light/362702/15 "2006-06-30T05:40:30Z")
> [@Ring](#): > > When waves destructively interfere it’s just like they were never emitted. That sounds suspiciously like a violation of conservation of energy. ---
**Author:** ![Mangetout](https://sea3.discourse-cdn.com/straightdope/user_avatar/boards.straightdope.com/mangetout/32/19_2.png) [@Mangetout](https://boards.straightdope.com/u/Mangetout)\ **Post date:** [June 30, 2006, 7:39am UTC](https://boards.straightdope.com/t/question-about-light/362702/16 "2006-06-30T07:39:42Z")
> [@TJdude825](#): > > 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.