To be pedantic, photons never accelerate. They only ever travel at one speed in one direction
And as they’re massless, photons do not experience time. Regardless of how far a photon travels, from its perspective, the journey takes no time.
A photon would somehow experience the big bang, the heath death of the universe and everything in-between all at the same time.
They don’t accelerate, but can travel at different velocities in different mediums.
For example light travels faster in air than in water and fastest in a perfect vacuum.
In aggregate, yes, but any individual wave of light is still traveling at c. You get the appearance of a slower wave because secondary waves are generated that cancel the original one in such a way that it makes a combined wave that appears to be slower.
Not quite.c is the speed of light in a vacuum. It’s more accurate to say c is the speed of causality.
Velocity/speed isn’t very useful with photons either - its a wave-particle.
Light in changing mediums is a separate but related phenomenon. The photon essentially doesn’t continue on its same path, it gets absorbed by the particles in the medium. This leads to changing states (of usually an electron in an atom) which may emit another photon, remain stable but increase the atom’s kinetic energy (I can’t remember how likely that is, if at all), or it may eject the electron, ionizing the atom. In any case, the state changes, because the whole system (the atom, electron, and photon) can’t have net energy gain or loss.
That was always my assumption about why it happened, but it turns out that’s not the case at all: https://youtu.be/CUjt36SD3h8
I believe they still travel at the speed of light, but are regularly absorbed and re-emitted in a way that makes the effective speed less than c.
