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Name_Unknown

Physics and Light

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Have a question for those who know Physics. What happens when lights travelling in opposite directions collide? Say I have two spotlights shining at each other. What happens when the two light beams collide? What I mean is what happens to the photons. Thanks for the help.

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Under some circumstances you may have interference patterns due to the superposition of the two beams of light. That means that in some regions of the beam it would actually be "dark". But in general, nothing special happens - photons do not bounce off each others, they are force carriers, not material particles.

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Quote:
Original post by John Schultz
Wave-particle duality.


Yes, I understand the duality of light (somewhat).

So, if light acts like waves and particles (we know this), both waves and particles will collide. So why won't shining two lights into each other make a collision (in the light)??? That is quite confusing.

Ok, yes well light always moves at the same speed. But it should be able to collide with other light as I can see. Why isn't this true? Isn't when the two lights cancel out when they have collided??

But I have another question. If I am standing in a field such that the light around me is being routed, would I be able to see? That is why I asked this stupid question. If I project light forward (in this imaginary field) will it hit the light coming at me (from outside the field) so would I be able to see?

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When traveling through air, it would appear photons act mostly wave-like (though there is some interaction with the gas that makes up air, and when interacting with matter, photons can behave more like particles). Thus the photons interact but do not collide. Given perfect inverse waves (using lasers), light can be cancelled out.

Photons
"Photon Collisions"
Failures of Classical Physics
Holography

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yes it's a wonderfull thing the complimentary nature of light. one thing I thought was cool that demonstrates the wave nature of light is those 3-d pictures of people that they carve into glass. you know you see them in the mall and the pircures are made out of many little bubbles burned into the glass chunk. they do this by shining two lasers into the object. each laser on it's own doesn't have the power necessary to burn glass, however, when the two light path's intersect the two beams combine constructively, increasing intensity and burring a small bubble.

just though it was a neat practical application of the wave nature of light. computer graphics however, rarely simulate this. we use geometric optics (photomorty) to describe the bulk of light's behavior.

to answer a few of your questions

we don't generally detect interfearence by light traveling in oposite directions, because we don't have the proper light to observe it. the light needs to be highly correlated. white light is really just abunch of photons at varying wavelengts, we can usually only observe interfearence when light is monocromatic(of a single wavelengh) the only devices that do this are lasers. so experiments with white light or even a red flashlight won't cut it. when photons interact with eachother, they do so obaying the wave nature. that is they do not bounce off eachother. when they interact with other matter, (such as electrons for instance) they obay the particle nature of light and transfer their energy to the matter. this is what is called the photoelectric effect in this sense the can be thought of as force carriers they can change the momentium of an electron. the photoelectric effect explains electron scattering and photon phase shifts when light hits a surface. in this case we can think of the photons as being able to transfer momentium because photon electron interactions change the electrons momentium. the angle of scattering given by compton-einstein formulas use momentium transfer concept, except it's relativistic momentium and not the normal mv.

when two phtons interact one of two things happen. first they do not "collide" they do not scatter off eachother. if we treated them as particles, they would collide and bounce off eachother causing them to deviate from their staright line path. this doesn't happen. what does happen is interfearence. two waves can combine constructively, or destructivley. and the exactly what happens depends on how they combine. when they combine destructively you have a decrease in intensity and vice versa when they combine consttructively. the math that dictates this is simply simple sin/cos funcitons. really neat actually. if you want to learn more about this, study the classic "double slit" experiments. this is one way to get light to correlate enough to produce interfearence patterns. it's really hard to do. it's easy to get light to be the same frequency, but it's damn hard to get two beams of light to be in phase with eachother.

tim

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You can do the double slit experiment with a laser pointer (5 quid from the local market), a piece of glass, some black poster paint (which you coat the glass with) and a razor blade (with which you cut two slits through the paint).

It's not all that hard unless you are trying to make the laser yourself :)

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Quote:
Original post by Name_Unknown
Hmm, that is interesting, shows you how bad my physics knowledge is.
I thought photons were particles like electrons.
They are. But photons are bosons, while electrons are fermions. Bosons do not obey the Pauli exclusion principle, which means they can behave in ways that seem non-intuitive to the idea of a "particle".

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I didn't mean hard hard lol. it's an easy experiment, it's just a lot of stuff going down to get light to that lvl of correlation is all. like the monocromatic laser and the slit and such.

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