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Air molecules can be oscillated by E field and re-radiate EM waves in different directions. However, if light is shined to a conductor the E field oscillate the free charges but the effect is to reflect the light or dissipated the energy by electrical resistance. Why there is such a difference?

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I'm guessing this is related to your earlier question, Will neutral particles be affected by EM waves?, and you're puzzled that Rayleigh scattering by air and reflection/refraction by solids and liquids seem so very different.

The answer is that in Rayleigh scattering each scattering object behaves as an independant scattering centre, so the scattering is incoherent. When light is reflected or refracted in a solid/liquid this is a collective phenomenon involving the collective electron density of many atoms and the scattering is coherent.

You do get coherent scattering in gases, which is why air will reflect and refract light just like a liquid or solid does. Rayleigh scattering is an additional (and small) incoherent scattering superimposed on the coherent scattering.

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  • $\begingroup$ my concern is that why air molecules scatter in all directions but conductors mainly reflects light and dissipate energy by resistance? $\endgroup$ – Kelvin S Aug 6 '15 at 15:01
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    $\begingroup$ @KelvinS: It isn't just conductors that reflect light. Glass reflects light as well, though admittedly conductors reflect more strongly. Reflection and refraction happens because the solid (non-)conductor scatters light in a coherent manner so all the scattered rays interfere. You only get constructing interference at the reflection/refraction angles. In Rayleigh scattering the scattering is incoherent so no interference occurs and the scattering is just random. $\endgroup$ – John Rennie Aug 6 '15 at 15:08
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Since the comparison is to air molecules I assume this is a question regarding very small conductors. The premise of the question is incorrect. This can be seen qualitatively by observing that the reason Rayleigh scattering by a dielectric particles occurs is that they reduce the electric field impressed on the particle. For a perfect conductor, the electric field is reduced to zero, effectively giving an infinite dielectric constant, so Rayleigh scattering is maximized by the conductive nature of the particle. A more rigorous presentation can be seen in section 2.1 of the paper http://www.physics.princeton.edu/~mcdonald/examples/small_sphere.pdf

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The solution for oscillating charges or plasmons has an evanescent field that exponential decays in the z direction (out into the "air"). This is currently being postulated as a way to subvert the diffraction limit. For more info regarding the lens : http://www.sciencemag.org/content/308/5721/534.

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