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Both reflection and refraction are processes where absorption and re-emission of electromagnetic waves occur, but they differ in the direction of re-emission. What factor particularly decides the direction of re-emission?

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First, some objects do both reflection and refraction.

Refraction is when light gets deflected when passing through the interface between two mediums of varying density. This happens with glass or air or water too. In this case the wavefront enters the new medium.

Now reflection is the change of direction of the wavefront at the interface between two mediums, when the angle changes in the opposite direction or close to that. In this case the wavefront does not enter the new medium.

Both cases are more elastic scattering. Where you say absorption-reemission, that is not true for most cases of refraction and reflection.

Your question whether the angle will be so that the wavefront will or will not enter the new medium is based on the molecular structure of the new medium. And in some cases, both happen at the same time. Some part of the wavefront will be reflected and some will be refracted.

But at the QM level, it is always the interaction between the photons and the atoms of the new medium that decide, so the molecular structure of the new medium.

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  • $\begingroup$ Can you explain me at the QM level? $\endgroup$
    – user197535
    Jun 6, 2018 at 4:08
  • $\begingroup$ " The reflection could be viewed as a two step process. The incident wave causes the electrons in the silver to vibrate like in an antenna. Though by vibrating they also emit the same light. So it's the electrons at the surface of the silver that reflect the incoming wave. As you mentioned the wave is part electric and part magnetic, but these cannot be taken apart since they are each others cause and effect: without one the other wouldn't be there either, and therefore it must reflect both parts. $\endgroup$ Jun 6, 2018 at 5:04
  • $\begingroup$ That silver (and all metals) don't distort is due to the fact that they are also very good conductors. This prevents the electromagnetic waves from entering the object. The boundary conditions which must hold (from being an conductor) result in the perfect reflection and that the resulting angle is equal to the incident angle. $\endgroup$ Jun 6, 2018 at 5:05
  • $\begingroup$ Similar boundary conditions are there for non-conducting materials like plastic and glass. These similar conditions result in reflection of glass and the shine/reflection on other smooth surfaces (though there can be other causes too). Also Snell's law would follow from these boundary conditions. $\endgroup$ Jun 6, 2018 at 5:05
  • $\begingroup$ In contrast to conducting materials it is possible for electromagnetic waves to enter non-conducting objects. As a consequence part of the incoming wave is transmitted into the material. The propagation or dampening of the wave through the material is largely dependent on the properties of the material. Some materials like glass hardly dampen the wave and you can see through them, while others like most plastics dampen them and thus are opaque. " $\endgroup$ Jun 6, 2018 at 5:05

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