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
It is best to not think of photons as little particles but as a ripple in a field (like a ripple in a pond).
That is to say, most of the properties of light are a result of it being a wave, and as a wave, the speed mainly depends on the medium itself.
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.
When a photon enters a medium, the fact that it’s an electromagnetic wave becomes crucial.
There’s a complex back-and-forth interaction between the photon and the charges in the medium (primarily electrons), where the photon makes them wiggle, and that wiggling in turn makes waves in the electromagnetic field that interact with the photon.
The result of that is that the wave that travels through the medium isn’t quite the same photon that entered, but once it leaves all that interaction stops and it’s the old photon again, because none of those interactions can actually permanently change the photon.
Think of water molecules as a bunch of mirrors. When light passes through and hits that H2O, it bounces around, but it never slows down. When it gets back out on air, there are way less mirrors to bounce against so it travels freely.
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