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When we derive refraction and reflection laws for a generical plane wave on a surface, we say that reflected and refracted must be in phase with the incident wave. Why a medium cannot do a sphased reaction beside incident wave? Sorry for my trivial question.

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Is this title a typo? I don't know what the word 'sphased' means. – Andrew Apr 7 '11 at 6:33
up vote 3 down vote accepted

It is possible. Take a look at phase conjugated mirror on Wikipedia.

Also, phase shifts do happen on reflection from flat surfaces -- all the time. It's called the Goos-Hänchen effect. It's just very very small, so that's probably why you ignore it in your derivation.

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Thank for the answer! Phase conjugated mirror is optically non linear: do you mean that this effect is a non linear effect? If yes, why in linear conditions you haven't phase shifts in reflection? – Boy Simone Dec 16 '10 at 19:04
A phase conjugated mirror's effect is non-linear, but the Goos-Hänchen effect takes place in linear media. It has to do with the penetration depth of the wave into the medium, if I remember correctly. – ptomato Dec 16 '10 at 21:58

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