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I was talking to a colleague about optical scattering from a metallic nanoparticle, and we had a very simple doubt. If you have a particle that's small compared to the illuminated area, you can use Mie theory to calculate the scattering you'll get. However, if the particle is very big, you have a mirrored ball and light will be reflected from it. We don't call that scattering.

Do we call the phenomenon "reflection" when we have "scattering" from a surface that's bigger than the illuminated area?

Or maybe it has to do with the polar intensity diagram: if the scattered distribution is narrow we call it reflection. However, the term "diffuse reflection" contradicts this idea.

NOTE: I understand that all these cases are just the solution to Maxwell's equations, and that we assign different names for different kinds of solutions. But my question is about how we name this processes.

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I believe the terminology of reflection is used for the case where the classical particle limit applies, so when the wave frequency is high enough so that the wavelength is smaller than the reflector, and a wavelength-scale patch of the reflector is approximately flat.

For particles of size comparable to the wavelength, even if they have pure Dirichlet reflecting boundary conditions on the surface. When the scattering is of a long plane wave, the forward scattering amplitude must removes intensity from the wave to account for the reflected energy. This means that there is just as much diffraction as scattering, a phenomenon discussed in Peierl's book "Surprises in Theoretical Physics".

When the wavelength is small, the scattering object is big compared to the wavelength and to the wavepacket size, it will remove all the intensity from the forward wave, making all the energy in the wavepacket reflect. In this regime, you call it reflection of a wavepacket, and the dynamics might as well be reflection of a particle. If you have a wide wavepacket, you can decompose it into a part that reflects and a part that just misses the scattering object, with the only diffraction close to the boundary. It is the geometric optics limit.

It is possible that this usage is inconsistent. I personally don't think anyone gets confused when you call diffractive reflection-like scattering from a small object "reflection", even though it doesn't follow the geometric optics trajectory.

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