# What tells a photon what to do?

1. Aug 30, 2004

### Metallicbeing

What is it that makes a photon find its way straight through transparent materials, but reflect away from materials with polished surfaces? Photons are absorbed by electrons, then re-emitted. What determines the angle of re-emission?

2. Aug 31, 2004

### pervect

Staff Emeritus
Maxwell's equations determine how an electromagnetic plane wave reflects and/or refracts at an interface between two media.

Quantum mechanics has to be equivalent to classical theory in the limit for a large number of photons.

The actual description of why photons do what they do involves some oddities, like the photons taking "all possible paths", and interfering with themselves. Feynman discusses this in "QED: A strange theory of Light and Matter", which is a pretty good popular book, if you don't mind not being able to actually calculate anything after reading it.

(ps: as good as it was, I found not actually being able to calculate anything *was a serious drawback, I found out by experience that an explanation that doesn't let you calculate something turns out to be a bit illusory).

3. Aug 31, 2004

### Chronos

4. Aug 31, 2004

### marlon

Pervect is referring to the path-integral-formalism of Feynman. The probability that an object moves from point A to point B is calculated as the sum of the probabilities of all possible paths between A and B. This SUM is really an integral when an infinite amount of paths are considered with a distance between them that evolves to zero.

In the double-slit experiment you have two openings, so the path integral is a SUM. But when you have an infinite amount of openings with distance zero between them and the openings themselves have a radius that goes to zero, the path integral becomes an integral instead of a SUM.

regards
marlon

5. Aug 31, 2004