What is light’s behavior in a medium?

Question

I’ve been reading questions and answers to this topic and have conducted a few experiments as well to try and understand what happens to light in a medium. Somethings just don’t seem to make sense to me. I’m not requesting that all of the questions below be answered, even just one would be appreciated.

1. Why does the reemission photon travel in the same direction as the photon that caused the reemission?

2. How could the medium, glass, water etc. vibrate at visible light frequencies. Seems too fast.

3. What info do the photons pass along the way that allows for the photons to exit the material in the same direction as those entering the material?

4. Is there a physical cause for the reduced wavelength. I know it is necessary for conservation of energy, but how is the wavelength actually decreased?

Answer 1

I will answer your questions but first we need to clarify:

there are three things that can happen when a photon interacts with an atom:

1. elastic scattering, the photon keeps its energy and phase and changes direction

2. inelastic scattering, the photon gives part of its energy to the atom and changes direction

3. absorption, the photon gives all its energy to the atom, and the valence electron moves to a higher energy level as per QM

In the case of glass, this is elastic scattering, this is the only way (just like with a mirror) to keep the photons' energy and phase an to create a mirror image.

Now in the case of glass, the direction is the same as the original (in the case of mirror it is opposite), but the photons inside the glass are traveling perpendicular (or at a different angle then the original) to the glass's surface, and they are regaining the original angle of travel when exiting the glass.

Now your questions:

1. the direction does change when the photons enter and exit the media. What is the same is the angle of refraction when entering and exiting.

2. it is not absorption.

3. it is not only the photons that make the angle of refraction, but the molecular and atomic structure of glass (the difference between the two medias)

4. what gets reduced is the speed of the wavefront in the denser media. each photon still travels at speed c inside the media (since photons always travel in vacuum).

Answer 2

By mentioning photons in your question, you implicitly require a quantum description of the process. It the must fundamental level the quantum decription of the interaction of a photon with anything else is given by quantum electrodynamics (QED). In QED the photon is always absorbed and re-emitted.

However, it would be extremely cumbersome to analyse the propagation of a photon propagating through a piece of glass, using QED in its most fundamental way. It would require coherently summing up all the different possible paths that a photon can take scattering from atom to atom (i.e. by being absorbed and re-emitted by each of these atoms). For that reason we use some approximations, such as effective medium theory.

What we can also do is to say that a single photon is just a single excitation of the wave function for the electromagnetic field. If there are no nonlinearities, then the behaviour of the wave function and that of the classical optical field is described by exactly the same equations. For that reason, we can know exactly how a photon propagates through a piece of glass by studying how a classical field propagates through a piece of glass. Assuming that you already know that, you also already know what a photon does.