system
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human
What happens to the photon when light gets slowed down while passing through a non-vacuum substance? If it gets absorbed and re-emitted by atoms, how can its direction remain the same?

There are two claims that I believe to have learned and that seem confusing to me.

One is that "A photon **always** moves at light speed."

The second is "A photon **always** moves in a straight line."

Maybe the first one is wrong though? The speed of light is only "the speed of light in a vacuum" isn't it? So what happens if a photon enters a different substance such as water or air?

Do the photons themselves somehow slow down while passing through the substance in a straight line (apart from being slightly redirected via refraction when entering from one homogenous substance into  another)? Or does the "light" (made up of photons) slow down because the photons spend the additional time wildly bouncing around inside molecules/atoms before reaching their initial goal point? If the latter is the case, how can one claim that "photons always move in a straight line" through a non-vacuum?

As I understand it, photons get absorbed by electrons in an atom which then jumps up an energy level (not sure how this is called in English) and then another photon gets re-emitted as the electron falls back down closer to the atom's nucleus. Surely the second photon isn't leaving the atom in the exact same direction as the first photon entered it, right?

Sorry if this is a stupid question, I'm just a bit confused here.
gpt
Let's stay away from the word photon for a few reasons: (1) it can mean different things in different situations (it can refer to a single vacuum eigenstate of the quantum EM field or it can refer to a superposition of states), and (2) this word inevitably misleads beginners into thinking light is made out of little mechanical balls. Light is quantum. As such, it is a little like waves and a little like particles. When traveling, light is more like waves and less like particles. So let's rephrase your questions:

"**Does light always move at speed c?**" No. Light only moves at speed c in vacuum. In materials, light travels more slowly. This is because of a wave effect known as refraction. If you insist on seeing light as a collection of photons, you have to realize that the basic picture of photons describes them as quantum bundles in vacuum. Once you are in a material, you can't use the basic picture anymore. The photon couples to the material and becomes something more complex which does not travel at c. The quantum particle approach is math-heavy and not very enlightening, so it's best to just treat light as waves that slow down in materials. Describing refraction as an absorption and re-emission of photons is simply wrong. Photon absorption is a resonant effect (a photon resonates with an electron that has a matching transition energy, causing the photon to be destroyed and the electron to transition). Refraction is an off-resonance effect, meaning that in refraction, light interacts with a whole bunch of electrons at once in a wave-like way without resonating with them too well. Light can be absorbed and reemitted by electrons. But that's not refraction. That's fluorescence, thermal emission, or even laser radiation. 

"**Does light always travel in straight lines?**" No. As a wave, light experiences diffraction, meaning that a portion of the light bends away from forward straight-line motion. You can't really describe diffraction using a purely particle photon picture since diffraction is a wave effect. Light only travels in straight lines to an approximation, known as the geometric approximation.