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Mirror has a silver coating which helps it reflect all colors of light. From my classical understanding, silver atoms reflect light by absorbing it and then emitting it at the same angle to the normal.

From QED, the angle of reflection is basically the result of application of principle of least action, but it doesn't rlly explain what happens at/within the atom (from what I gathered). So do we know what rlly happens within the atom to produce the exact same angle?

Also, how does mirror emit the exact same frequency of light, and how does it reflect all frequencies of visible light? Silver atoms can only absorb specific wavelengths and re emit them. So how does it"reflect"all kinds of colors?

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  • $\begingroup$ Light creates dipoles at the surface that emit same frequency. For metals it will be currents and the plasma that will produce the reflection. If you ask why same angle it is conservation of momentum. Some specific materials will absorb some of the colors and you will see the complimentary colors. $\endgroup$
    – user591849
    Mar 10, 2019 at 8:28
  • $\begingroup$ @user591849 What plasma are you talking about? $\endgroup$
    – PM 2Ring
    Mar 10, 2019 at 9:19
  • $\begingroup$ A gas of free electrons. The plasma frequency determines mostly which spectrum is reflected or absorbed transmitted and what is the color of the metal. $\endgroup$
    – user591849
    Mar 10, 2019 at 10:29

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Silver atoms can only absorb specific wavelengths and re emit them. So how does it"reflect"all kinds of colors?

The answer is that it is the conduction electrons in a metal which are responsible for reflection.
The influence of the bound electrons is usually swamped by the effect of the conduction electrons.

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There there are many ways you can explain this and people have answered this too.

I am sharing a link to a Similar Question .

Atoms absorb photons and remit them with same frequencies nearly but just with a Phase lag of π/2 . To explain the law of reflection it can be said that light is emitted in all directions but it's because of the interference that all other waves interfere destructively and light wave with same angle only interferes constructively and happens to be useful to us thus law of reflection holds true.

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An isolated single atom is small compared with light wavelength and so ”scatters” light in all directions if at all. Also one atom cannot normally know which way the “normal” to the mirror is. However Interference of all the scattered waves from the different surface atoms will be constructive in the specular reflection direction. This is largely independent of surface atom spacing and basically the same if scatter from surface conduction electrons... always get constructive interference in specular reflection direction if surface smooth on light wavelength scale 0.4 - 0.7 micrometers for light wavelength.

Note single atom doesnt need to scatter light in the same form as does a lump of the material. Especially for metals like Farcher says. Also you don’t need QED unless you’ve got gamma rays from a supernova or cern and where we might have enough energy to produce antimatter! ... for visible light you just want to use wave theory, electromagnetism and basic quantum mechanics with some atomic theory, not subatomic! Nontheless, the least action principle does have a version for mirror in the fact that a reflected beam does take a shortest route and least time under some constraints.

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