3 upvotes, 2 direct replies (showing 2)
View submission: Ask Anything Wednesday - Physics, Astronomy, Earth and Planetary Science
How does refraction work? My current understanding is: one end of the lightwave hits a different medium and slows first. This turns the "front" of the wave towards the slow side.
But light is photons right? How does the photon at the far end of the line know the first one has slowed down?
Comment by tredlock at 14/09/2023 at 01:57 UTC
4 upvotes, 0 direct replies
In the classical regime, where light is described by the source free Maxwell equations, your understanding is basically correct. In a medium, which has a higher impedance than free space, the waves slow down. In order to not have a discontinuity at the interface, this necessitates the wave changing direction.
Now as to the question of a quantum description of classical optical effects, I will try to boil down what’s a notoriously conceptually difficult area. Photons properly arise from quantum electrodynamics—our current best theory of electromagnetism.
In QED it’s a little less clear how to interpret refraction, but I think for a layperson Feynman’s description is the best. One of the ways to formulate a quantum field theory is the path integral approach. In this approach, one essentially sums over all the possible classical paths to arrive at a probability amplitude, and paths that are more favorable classically contribute more heavily to the amplitude. Since light classically obeys Fermat’s principle (light takes the extremal path), the paths that are more favored in a medium are those that follow the refracted path, and thus give rise to a higher probability amplitude.
To answer your question more directly, the photons don’t have to know about each other. The fundamental object is the quantum field itself (the object you get from doing this path integral, or sum over classical trajectories), which spans spacetime and behaves analogously at interfaces like classical fields.
Feynman’s book QED: The Strange Theory of Light and Matter is a great read if you want to learn more about the quantum nature of light.
Comment by Crazy_questioner at 14/09/2023 at 01:18 UTC
2 upvotes, 0 direct replies
The effects of reflection, refraction, and diffraction are all single -photon events that occur when each encounters an atom inside a medium. That medium will have a general bulk property that roughly changed in the same direction.
Diffraction has a bit more to it than that but i don't think that's what you were asking about.