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## what is the cause of reflection?

 Quote by sophiecentaur This link http://en.wikipedia.org/wiki/Phase_(waves) should be a help, with or without the maths; the pictures are not bad in showing the gist of what a phase relationship means. Try that.
simply speaking ... out of phase means that the waves different phases ( or angle of phase shift is other than 0 degree).... is that right?

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 Quote by A Dhingra thanks... well one more thing ....you have mentioned vibrational modes...how do they affect the reflection... rather collision..... is it that the amplitude of vibration affects the possibility of collision?
If there is a vibrational mode of the right energy available then the photon will be absorbed instead of scattered. This is the principle behind infrared spectroscopy since the vibrational modes fall in the infrared energy range.

http://en.wikipedia.org/wiki/Infrared_spectroscopy

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 Quote by A Dhingra simply speaking ... out of phase means that the waves different phases ( or angle of phase shift is other than 0 degree).... is that right?
Simply speaking, waves with different phases have peaks and troughs at different times. You could 'see' this if you were to take a picture of the two waves - the peaks and troughs would appear in different places. Otherwise you could look at their variations in time (such as with an oscilloscope), in which case the peaks and troughs would be seen, also, to occur at different times.
Two signals (not necessarily waves) of the same frequency can also have a 'phase difference' between them. Two kids on swings of equal lengths may be going at the same rate but not coincide in time / phase.

The use of vectors (or, rather, Phasors) is a good way to show their phase relationship by displaying them as a 'frozen' circular motion. This method can show their amplitudes and phases and make it easy to show the result of adding the oscillations together. Wikkers is bound to have an entry on Phasors.

 Quote by Grep Kind of difficult to explain in a concise way. I suggest you read Richard P. Feynman's "QED: The Strange Theory of Light and Matter". [..] He was a truly great teacher.
 Quote by sophiecentaur Don't you think it's demanding a bit much of someone who doesn't know what "out of phase" means, to tell them to read Feynman?
 Quote by sophiecentaur The use of vectors (or, rather, Phasors) is a good way to show[..]
For someone who is interested in how reflection works on the microscopic scale, but doesn't have a strong technical background yet, I think Feynman's little popular science book "QED" is indeed the best possible resource.

It captures the excitement of the science, and it conveys the true technical mechanism without misleadingly oversimplifying and without needing the technical jargon nor confusing math.

(Phasors, on the other hand, are such an abstract and esoteric topic among physics graduates nowadays, hardly the first concept I would recommend someone begin from..)

 So are you guys saying that reflection has to do with light having an E and B field . And it gets affected by the electrons in the material. What I always had trouble with is that light doesn't have charge so why would this affect the photon. And when I shine a laser into a B field it doesn't alter its path. I am probably missing something fundamental here.

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 Quote by cragar What I always had trouble with is that light doesn't have charge so why would this affect the photon.
There is more to EM than just the Lorentz force law. EM fields interact with the fields, not just with charges (see Maxwell's equations). Otherwise you couldn't have waves in vacuum.

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 Quote by cragar So are you guys saying that reflection has to do with light having an E and B field . And it gets affected by the electrons in the material. What I always had trouble with is that light doesn't have charge so why would this affect the photon. And when I shine a laser into a B field it doesn't alter its path. I am probably missing something fundamental here.
The problem is that you, and this seems to be common, are mixing in both classical and quantum ideas. Thinking about charges and currents and using that as an explanation for the classical field reflection is perfectly fine. The incident fields induce currents on the surface and interior of the material via the Lorentz force. These induced currents create secondary fields which cancel out the transmitted fields and produce the reflected fields. Thus, the total field is an incident and reflected field without any transmitted wave (assuming total reflection). In terms of photons however, we need to look at the energy modes of the material and the band structure all of which are determined using quantum mechanics. It then can become a problem of scattering (in the case of reflection) and/or of transmission into one of the supported modes of the problem (where the diffraction can be solved for example via conservation of momentum). One can also use the idea of a path integral to get the same results (whose classical analogue is Fermat's principle).

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