I while back I learnt that when light is incident on a dipole the dipole will scatter the light, and when it is incident on a material of a different refractive index then the light refracts. From the Ewald-Oseen extinction theorem, it seems that refraction is caused by scattering. So what is the distinction between scattering and refraction (i.e. when would we call something scattering and something refraction)? (sources would be helpful if you have them)
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asked Apr 4, 2016 at 14:17
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3 Answers
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Refraction occurs when a large number of dipoles scatter coherently. Each individual dipole scatters light in response to the incident radiation in (almost) all directions, but when you have a large collection of scatterers, each one scattering in many directions, you have to sum the contributions of each one in order to arrive at the total field. Each contribution interferes with every other contribution. When you do this at an abrupt interface, the result is reflection and refraction (and cancellation of the incident light, ala the Ewald-Oseen thm).
So the main difference is that scattering generally refers to small scatterers (having a size on the order of the wavelength), and refraction requires a large number of scatterers, and a clean interface.
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1$\begingroup$ What do you mean by "almost" in your second sentence: "Each individual dipole scatters light in response to the incident radiation in (almost) all directions"? Which direction(s) are excluded and why? $\endgroup$– BlaiseCommented Oct 29, 2018 at 22:17
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1$\begingroup$ @Blaise A dipole does not radiate in the direction of it's own alignment. That is the same direction as the polarization of the exciting field. That's because the dipole oscillates in that direction, so the polarization of the radiation must have a component in that direction. That's not possible for radiation traveling in that same direction. The polarization of that radiation must be perpendicular to that direction. $\endgroup$– garypCommented Oct 30, 2018 at 11:15
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Generally speaking, the first and main difference is that refraction happen upon transmission of the light, while scattering happen upon reflection of the light (namely, diffusive reflection, where the angle of reflection does not equal to the angle of incident).
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Angle of reflection is always equal to angle of incidence, even in diffuse reflection. A diffusive reflector can be approximated as multiple tiny reflectors not having the same orientation (their normals are not parallel).
