# Explaining reflection and refraction in atomic level

https://www.physicsforums.com/showpost.php?p=899393&postcount=4 [Broken] describes why photons are slower in some medium. After reading it, I've few more questions:

When a photon hits an electron, if it absorbs the photon and reemits, here's my questions:

1. How does refraction possible?
The electrons are moving around randomly. The photon that entered into the electron does not need to be reemitted in the same direction. But how are we able to see things?

2. How does reflection possible?
The surface may look flat, but in a macroscopic level. But for a photon, it just hit a round oblect (nucleus or electron). To me, the coefficient of reflection in terms of angle of incidence does not make any sense in a particle level. Mathematically, to have an angle, you need 3 points. 2 points in the reflecting surface, one point for the photon. With these 3 points, you could say the photon hit the surface in an angle. But for a photon, it hit just one electron. It does not care about the other electrons in the surface. In that case, there is no 3 points, it is only 2 points. But then, how does the reflection work?

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## Answers and Replies

jtbell
Mentor
When a photon hits an electron, if it absorbs the photon and reemits,

The main point of ZapperZ's FAQ is that we are not dealing here with the interaction between a photon and a single electron, or even a single atom!

ZapperZ said:
When atoms and molecules form a solid, they start to lose most of their individual identity and form a "collective behavior" with other atoms. It is as the result of this collective behavior that one obtains a metal, insulator, semiconductor, etc. Almost all of the properties of solids that we are familiar with are the results of the collective properties of the solid as a whole, not the properties of the individual atoms. The same applies to how a photon moves through a solid.

A photon can't interact with a single free electron because energy and momentum can't both be balanced. The photon interacts with the structure.

A photon can't interact with a single free electron because energy and momentum can't both be balanced. The photon interacts with the structure.

I did not clearly get what was meant by interaction by the structure. Ultimately, the photon has to hit one of the atomic particles, right? I'm talking about a photon hitting an electron in an atom, not a free electron.

To me, explaining things in a macro level (behavior in structure level, in this context) is just for ease of explaining complex cases. But everything should be explainable in the particle level too. Right?

Claude Bile
Ultimately, the photon has to hit one of the atomic particles, right?
Photon transmission is not a localised interaction like atomic absorption, so the pure particle description of a photon in this scenario is a very poor one. Photon transmission is primarily a wave-like phenomenon, so a classical wave description of the photon is more apt.

Back to your original post;
jobyts said:
1. How does refraction possible?
The electrons are moving around randomly. The photon that entered into the electron does not need to be reemitted in the same direction. But how are we able to see things?
This is one of the main arguments AGAINST the whole misconception of photon transmission through a solid being a series of absorption/re-emission events. Simple conclusion - the photon is NOT being absorbed and re-emitted.
jobyts said:
2. How does reflection possible?
The surface may look flat, but in a macroscopic level. But for a photon, it just hit a round oblect (nucleus or electron). To me, the coefficient of reflection in terms of angle of incidence does not make any sense in a particle level. Mathematically, to have an angle, you need 3 points. 2 points in the reflecting surface, one point for the photon. With these 3 points, you could say the photon hit the surface in an angle. But for a photon, it hit just one electron. It does not care about the other electrons in the surface. In that case, there is no 3 points, it is only 2 points. But then, how does the reflection work?
This is actually not a bad question, however it becomes somewhat overshadowed by the fact that it is quite impossible in practice to obtain a perfectly flat surface anyway. Putting it crudely, as long as the wavelength of the incident radiation is a good deal larger (10 times larger or thereabouts), what you get is the wave interacting with the "average" surface (i.e. the surface with all the "kinks" removed) plus a small amount of scatter. The bigger the kinks, the more scatter one gets.

Claude.

I'm talking about a photon hitting an electron in an atom, not a free electron.

I think you're confused, a photon does not "hit" an electron in an atom, it interacts with the atom, molecule, or extended structure, and it can interact in a variety of ways. Pick up a copy of Feynman's little book QED, it will answer most of the light interaction questions you've posted in a very straightforward intuitive way, and you can read it in an evening.