Naked Science Forum

Non Life Sciences => Physics, Astronomy & Cosmology => Topic started by: alexashcroft on 25/09/2017 16:17:54

Title: What causes light reflection?
Post by: alexashcroft on 25/09/2017 16:17:54
Tomusange would like to know:

What causes reflection?

What do you think?
Title: Re: What causes light reflection?
Post by: chris on 29/09/2017 13:56:11
bump!
Title: Re: What causes light reflection?
Post by: kazbert on 29/09/2017 20:49:55
Cool question!  It is leading me to dig a little deeper into a phenomenon that I have only ever considered on a macro level.

It's complicated.  Not only is there more than one mechanism for reflection, but different substances are transparent/reflective/absorptive to different frequencies along the EM spectrum.  Different substances also absorb differing amounts of the incident energy. A polished silver mirror will absorb very little of the incident visible light compared to a polished steel mirror.  Both will reflect in the majority of the visible spectrum, but the reflected light from a steel mirror will be noticeably dimmer (lower energy) compared to silver. 

These things I already knew. 

A detail that was new to me is that the photons are not just bouncing off the reflector.  They are being absorbed and then re-emitted.  The articles I've found so far haven't described why (in the case of a sliver mirror, for example) the silver atoms are able to re-emit a photon that is exactly the same frequency as the absorbed photon.  I know energy must be conserved, but the electrons in a high-energy, excited atom can only fall back down to specific shells, meaning the energy in the emitted photon corresponds to a specific frequency. 

And by what atomic-level mechanism does the Law of Reflection hold?  On the atomic level what is happening on a specular surface that causes the angle of reflection to be the same as the angle of incidence?  Why aren't all surfaces diffuse? 
Title: Re: What causes light reflection?
Post by: chiralSPO on 29/09/2017 21:09:03
A detail that was new to me is that the photons are not just bouncing off the reflector.  They are being absorbed and then re-emitted.  The articles I've found so far haven't described why (in the case of a sliver mirror, for example) the silver atoms are able to re-emit a photon that is exactly the same frequency as the absorbed photon.  I know energy must be conserved, but the electrons in a high-energy, excited atom can only fall back down to specific shells, meaning the energy in the emitted photon corresponds to a specific frequency.

Atoms within a metallic object cannot be considered individually. One must look at the electronic structure of the whole materials (metals have delocalized orbitals throughout the entire material). Instead of talking about "energy levels" we must talk about "bands." (and for metals specifically, there is no "band gap" between occupied and unoccupied levels, so they can interact with light over a very broad (and effectively continuous) range. https://en.wikipedia.org/wiki/Electronic_band_structure

I can't help with an atomic-scale reason for the angles of reflection (specular over diffuse), but my suspicion is that it has something to do with conservation of momentum.
Title: Re: What causes light reflection?
Post by: kazbert on 29/09/2017 21:32:34
"energy levels" we must talk about "bands."

That was helpful!  My own prior experience with excited metallic atoms was with vapors, not solids, so the phenomenon I dealt with was with individual atoms.  I agree, too, that a conservation of momentum is likely at play in the Law of Reflection.  Should have thought of that!  I'm getting old.  What I know I know well, but everything else is rusty.
Title: Re: What causes light reflection?
Post by: Bill S on 29/09/2017 22:26:39
    As I understand it, solid is composed of a network of ions and electrons fixed in a "lattice".  These have "collective vibrational modes", sometimes called “phonons”. They are quanta of lattice vibrations, and it is these vibrational modes that can absorb a photon.  So when a photon enters a solid, and it can interact with these phonons, it can be absorbed by the solid and then converted to heat. The solid is then opaque to this particular photon (i.e. at that frequency).  This is why some materials are more absorbent than others. 

Unlike the atomic orbitals which are discrete, the phonon spectrum can be broad and continuous over a large frequency range.  If a photon has an energy beyond the phonon spectrum, the solid cannot sustain this vibration, because the phonon mode is not available.  So the lattice does not absorb this photon and it is re-emitted but with a very slight delay when light is passing through a transparent medium. This appears to be the origin of the apparent slowdown of the light speed in that particular medium. 

It seems that this mechanism may also apply to reflection.
Title: Re: What causes light reflection?
Post by: saspinski on 30/09/2017 02:45:20
In the book "QED - The Strange Theory of Light and Matter" , Richard Feymann explains the reflexion by Quantum Mechanics, but trying a intuitive approach.

 Taking a point A as the source of the photons and a point B as the position of a detector, each photon has an amplitude of probability (ap) to reflect at any point of the mirror surface and be dectected. But except for the neighborhood of the point where the angle of reflexion is equal to the angle of incidence, the ap of reflecting at a given point cancels out with the ap of another point nearby. The ap's can be represented as vectors of the same magnitude, but pointing to different directions.

 The process could be understood  classicaly as an example of destructive interference, but it happens to individual photons and not only to continuous light waves! That correspond by the way to the shortest time of travelling, as can be proven geometrically, what means in classical mechanics that the Action of the light path is minimum.
Title: Re: What causes light reflection?
Post by: evan_au on 30/09/2017 08:20:59
Another way to look at metallic reflection is by magnetic induction.

An electromagnetic wave approaching a metal surface will induce a current in the free electrons of the metal, which will produce a magnetic field which is equal and opposite the magnetic field of the incoming photon (according to Lenz's Law).

This is effectively a wave propagating out of the metal surface
- in the same direction as the incoming wave (measured parallel to the surface)
- in the opposite direction as the incoming wave (measured perpendicular to the surface)

At the point that the cross (at the surface), all that is left is the wave propagating out of the metal surface, where the angle of incidence equals the angle of reflection.

Of course, real metals are not perfectly conductive, so the incoming wave is not perfectly reflected. But good conductors like aluminium and silver can give a good reflection in astronomical mirrors.