# B Is it true that QED explains the reflection of light?

#### fbs7

About a month or two ago I posted this question in the "Classical Physics" forum: if the light doesn't interact with an electromagnetic field, then which force explains light reflection in a mirror?

I didn't get a clear answer for that (besides advice to buy a book from Feynmann), so I went on googling that. The explanation I read (in Quora) is that light reflection is explained through absorption/re-emission of photons, and under Quantum Electrodynamics the angle of re-emission needs to be the same (ie complement) of the angle of incidence.

Now, I understand QED is the quantum view of the electromagnetic force, is that right? If so, it's so surprising! If I put a ray of light under a strong electric field there's no interaction, but if the light is absorbed then re-emitted by an atom, then there is an interaction explained by quantum electrodynamic.

Did I get the basics of that right? That is, electroweak is really the force involved with light's reflection in a mirror?

Related Quantum Physics News on Phys.org

#### PeterDonis

Mentor
About a month or two ago I posted this question in the "Classical Physics" forum: if the light doesn't interact with an electromagnetic field, then which force explains light reflection in a mirror?

#### PeterDonis

Mentor
if the light doesn't interact with an electromagnetic field
Why do you think light doesn't interact with an electromagnetic field? Classically (and you asked the original question in the classical physics forum), light is made of electromagnetic fields.

#### PeterDonis

Mentor
If I put a ray of light under a strong electric field there's no interaction
What do you mean by "no interaction"? What specific experiment are you thinking of?

#### PeterDonis

Mentor
The explanation I read (in Quora)
...is going to be a lot less detailed than an explanation in an actual textbook, or even the explanation in Feynman's QED book, which is a popular book for the lay person, not a textbook (although for a popular book it does a very good job of not saying anything that might be misleading or might need to be unlearned later when you have a deeper understanding).

#### PeterDonis

Mentor
QED is the quantum view of the electromagnetic force, is that right?
It's the quantum theory of electrons and the electromagnetic interaction. (Note that this is not the same as the electroweak interaction; the electroweak interaction is part of the Standard Model, which is more general than QED. QED can be thought of as a simpler approximation to the Standard Model that ignores all particles other than electrons and photons and all interactions other than electromagnetic.) But "interaction" here is more general than the usual interpretation of the term "force".

#### Heikki Tuuri

Classically, the reflection happens because the light wave, say, a coherent laser beam, makes electric charges in the mirror to move.

The oscillation of the charges creates the light beam which leaves the mirror surface.

The silvering in a mirror is made of metal because lots of electrons can freely move in metal. Transparent materials cannot have freely moving electrons.

QED (= Feynman diagrams) is about collisions of individual particles. It does not directly apply to a mirror because the mirror is a bound state of a huge number of particles. However, scattering of photons from electrons in QED casts some light on the physics.

#### vanhees71

Gold Member
QED is describing everything concerning the electromagnetic field and its interactions, not only collisions of individual particles. There's not only QED applied to scattering but also many-body QED, and it's heavily used in (theoretical) quantum optics.

To answer the OP: Reflection of electromagnetic waves on a conductor is due to the interaction of the em. field (and light is just an em. field with typical wavelengths in that range we can see with our eyes, i.e., about 400-800 nm, roughly speaking) with electric charges within the conductor. It consists of the superposition of the incoming ("external") field and the field generated by the accelerated charges within the conductor. The same holds true for dispersion in a dielectric. There you've refraction and also reflection, following the Fresnel equations (within the applicability of the usual linear-response approximation of light-matter interaction).

Concerning the scattering of light by light (Delbrück scattering) that's a higher-order quantum correction. The lowest-order contribution is from a box diagram with charged-particle lines running around, and the corresponding cross section is of order $\alpha^4$, i.e., very small. The effect has only been observed very recently ago by the ATLAS collaboration at CERN in ultraperipheral heavy-ion collisions (Pb on Pb).

#### Heikki Tuuri

Thank you to @vanhees71 for correcting my too narrow a view of QED.

Reflection of radio waves is even more classical than reflection of light. Radio waves which you generate with a radio transmitter are coherent electromagnetic waves.

The electric field of an incoming radio wave makes electrons move in metal. The oscillating electrons emit new radio waves which are the reflection wave.

https://physics.stackexchange.com/questions/248726/how-does-qed-explain-reflection

"Anna v" in the link above writes that in QED we must consider the electrons at the surface as a single field, from which a photon scatters.

Now I realize that the reflection of light and longer waves is best understood as a wave phenomenon. If we shoot X-rays or shorter waves at metal, it is more like particles colliding with other particles. The distance of atoms within metal is around 0.1 nm. A light wave is 500 nm. A light wave will feel the electrons collectively.

#### vanhees71

Gold Member
Sure, usualy everyday em. waves are either thermal states (more or less black-body radiation) or coherent states. Until recently it was not easy to provide true single-photon states. All started with Aspect's experiment who used a atomic transition cascade excited by a laser in the early 1980ies and lead to the first proof of the violation of Bell's inequalities and a confirmation of standard QED. Today the quantum opticians have parametric downconversion at hand with much more efficient single-photon (or rather entangled-two-photon) production rates.

The reflection from a mirror is described in QED quite as in classical electrodynamics. As long as the linear-response theory is sufficient, the operator equations are not more difficult to solve than classical Maxwell equations.

#### fbs7

What do you mean by "no interaction"? What specific experiment are you thinking of?
Hello Peter; my thought was that if you pass a ray of light through an extremely strong electric or magnetic field then the light will not be affected by it, because it has no electric charge, so it just keeps going straight through it.

Is that wrong? That is, if the electric or magnetic field is strong enough then it will affect light in some way?

#### fbs7

Classically, the reflection happens because the light wave, say, a coherent laser beam, makes electric charges in the mirror to move.

The oscillation of the charges creates the light beam which leaves the mirror surface.

The silvering in a mirror is made of metal because lots of electrons can freely move in metal. Transparent materials cannot have freely moving electrons.

QED (= Feynman diagrams) is about collisions of individual particles. It does not directly apply to a mirror because the mirror is a bound state of a huge number of particles. However, scattering of photons from electrons in QED casts some light on the physics.

Oh, wow! That's mind-blowing awesome!! I have to read each paragraph most carefully, this is amazing!

So light creates an electric field in the surface (maybe, I suppose that the photons are absorbed by the electrons that are moving in the surface, which get energized and move in some specific way, is that right?), then that electric field ends up causing the emission of new photons, and then that field disturbance vanishes when the light is re-emitted. That's awesome!

That sounds very much like the photo-electric effect, except for the re-emission - so the light reflection and the photo-electric effect are related?

Also, this sounds very much like when I throw a flat stone in a lake - it bounces out leaving waves in the surface, except that light doesn't leave waves in the mirror; for the light it seems to me, like, almost 100% of the energy gets reflected.

I stand corrected on "QED explains reflection"; my original curiosity was which of the 4 forces (electromagnetic, gravity, weak and strong) is involved with the reflection, so if an electric field is involved, then definitely light reflection involves electromagnetism, is that correct?

Finally, transparent materials do not have free flowing electrons - that's awesome!!! I'd never imagine that! That's why metals tend to look... metallic... I guess.

#### PeterDonis

Mentor
my thought was that if you pass a ray of light through an extremely strong electric or magnetic field then the light will not be affected by it, because it has no electric charge, so it just keeps going straight through it.
Classically, if the field is in vacuum, this is correct. The underlying reason is that Maxwell's Equations are linear, so in the absence of sources (which means light just travels as an EM wave), the EM fields in the light ray will simply superpose linearly with the field produced by your experimental apparatus, and they won't affect each other.

However, light obviously can be affected by traveling through materials (for example, refraction), and classically, this is an interaction between the light and the electromagnetic fields produced by the atoms in the material. In terms of Maxwell's Equations, sources are present and so pure source-free EM waves are no longer solutions.

Quantum mechanically, as @vanhees71 mentioned, there are higher order corrections that can allow even light in vacuum to interact with itself. But those corrections are extremely small and have only recently been observed experimentally.

#### fbs7

Wow, thanks everybody for their answers! Quite spectacular information, I'd never imagine it! Thanks all!