[Ben Mugged]

Cerenkov radiation occurs when a particle plows into a substance such as water and momentarily exceeds the speed of light in that medium

Then the excessive energy that has to be disposed of during the velocity change generates radiation? Is this true of all mediums? When light transitions from water to air to space does it accelerate? Where does it get the energy to accelerate? Does it absorb it from the medium it is leaving?

[RightWhale]

It is not the light that generates Cerenkov radiation but the particle. The light will be generated while the particle exceeds the speed of light in the medium.

[Ichneumon]

Then the excessive energy that has to be disposed of during the velocity change generates radiation? Is this true of all mediums? When light transitions from water to air to space does it accelerate? Where does it get the energy to accelerate? Does it absorb it from the medium it is leaving?

IIRC, light doesn't actually "slow down" in a medium like water, it just takes longer to pass through it because it makes multiple short "pit stops" along the way, but still travels at the same constant velocity between stops.

[spunkets]

"When light transitions from water to air to space does it accelerate?"

No. The velocity of light in any medium, including the vacuum, is fixed. It is fixed, because of the electromagnetic and spacial properties at those coordinates. The velovity of the electromagnetic energy is given by:

c = 1/sqrt(e * e_o * u * u_o)

where e_o and u_o are respectively, the permittivity and permeability of the vacuum. e and u are respectively, the dielectric constant and permeability of the material if present. They represent an interaction strength at each point. That interaction takes time, so energy moving from one point to the next requires a delay, or interaction time at each point. Light emerging from one medium to another doesn't accelerate, it just encounters different interaction times.