# How does a photon 'know' when to reflect in case of refraction? [closed]

In thin-film interference and in glass fibers refraction is involved. Depending on the angle of which a light ray enters the material it will be reflected or transmitted. The refraction depends on the different materials like air and water, glass or oil.

But as far as I know the reflection happens within the material of the thin-film or the glass. So from that point of view that photon haven't 'seen' or 'felt' the material on the other side of that material which refraction index would reflect them. So when a photon is reflected before he 'knows' the other material (as he is reflected by the atoms of the same material as he went in) how does he 'know' that he 'has' to bounce back?

## closed as unclear what you're asking by John Rennie, honeste_vivere, Diracology, ACuriousMind♦, CuriousOneJul 23 '16 at 1:38

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• A light ray isn't simply a bunch of photons. In fact photons are a very poor way to describe a light ray. If you insist on using photons you'd have to describe the ray as a coherent state on many delocalised photons. Being delocalised the photons feel the whole system the light ray is passing through. – John Rennie Jul 22 '16 at 10:41
• In addition to @JohnRennie 's comment: you are asking for a quantum theory of reflection/ refraction. This is usually well explained by QED. The reason for this is that you need to model the photons as being in a coherent state. For refraction, the angle is defined of refraction is such that the photon beam remains coherent AND the wavefunction is continuous at the interface. This is should give you a quantum formulation of Snell's law. This holds only for a range of angles. Outside this range, coherence requires negative angles which means that the light beam stays in the same medium. – user106422 Jul 22 '16 at 10:58
• Also note that the reflection/ refraction laws depend on the dielectric constants of that medium which can be computed in a quantum theory. – user106422 Jul 22 '16 at 10:59
• @Eh-whaaa We don't need a coherent state to describe reflection and refraction. Very low intensity beams ("one photon at a time") also obey Snell's Law. Indeed, the picture of a photon as a particle like a ball is a very poor model. I'll just repeat the point that in the quantum theory, the excitation of the field ("photon") is delocalized, and does "know" the other material. – garyp Jul 22 '16 at 11:05
• Possible duplicate of How are classical optics phenomena explained in QED (Snell's law)? – ACuriousMind Jul 22 '16 at 23:05