Does one side of a glass slab get cooler and another side get hotter when the light passes normally through it? Let us have a glass slab and we illuminate it by a light beam impinging normal to the slab and passing through it. Since the speed of light is slower inside the glass it loses its energy and when it exits the slab it gets up to speed of light in free space again. In this energy exchange the input side of the slab gets hotter and the output side gets cooler.
Is the argument  above valid?
 A: The speed of light in the medium decreases because there is a delay due to the absorption and emission of the light wave by atoms. This delay has a different value for different atoms.  The atoms return to their basic state by emitting energy.
However, the speed of light in a medium largely depends on the density - the distance of atoms in the medium.
The passage of light through the glass causes reflection and etc.  However, the absorbed energy will sooner or later radiate it, for example in the form of heat. Atoms always want to return to their stable state.
How much energy from the light wave has been reflected or converted into another form of energy can be verified, among other things, by measuring the brightness of the light before and after it enters the medium or by checking the spectrum of the light wave.
You can trap an atom by laser light to lower its energy: https://en.wikipedia.org/wiki/Lene_Hau

In this energy exchange the input side of the slab gets hotter and the output side gets cooler.

However, the process of simultaneously increasing and decreasing the energy of the same type of atoms by a common light wave does not take place. I don't see any grounds for such a process.
Theoretically, if the frequencies of the absorbed and radiated waves on atoms were changed, the ratio of these new frequency to the frequency of the next atom would be appropriate. Such a process could take place. However, glass does not have such properties.
The description of each phenomenon can be boiled down to the energy ratio - the frequency of the source and the target.
Sorry for My English.
