eli5: Why doesn’t photons loose velocity after going through a denser medium?

Question

If photons go through air>liquid>air, then why it’s velocity remains the same before entering the liquid medium and after exiting the liquid medium.

In: Physics

Answer 1

Photons are massless, so they don’t move using kinetic energy that could be lost in the denser medium. When photon enters denser medium, it indeed slows down, not because of energy loss, but because of interaction with medium’s atoms. Once photon exits the medium, it goes back to move with its previous velocity, since it didn’t lose energy.