What is the acceleration rate of light?

Good question. Light does not accelerate; it is generated (by decoupling from charged particles) at the speed of light C in a vacuum. So it's always C.

And in case you think of it, the refraction index n = C/c; where c < C is the average speed of light through some medium other than a vacuum. In which case, as the light emerges from that medium, doesn't it accelerate from c to C > c upon the exit?

Nope. Here's why.

The photons never slow down to c; it's their average speed, not their instantaneous speed that is represented by c < C. Here's how that works.

To cross an increment of free space dS, it take t = dS/C time. But to cross that same distance filled with stuff, like matter, it takes longer T > t. It takes longer because it takes time for the photons to couple and then uncouple with the electrons making up that matter. I call it dwell time. So each time a photon couples and another is emitted, that adds more dwell time to the transit time across that space.

So we have T = dS/c > dS/C = t. In which case T/t = dS/c//dS/C = C/c = n that refraction index. And this leads to the conclusion that the average speed of light through a medium will always...always...be slower than the speed of light through a vacuum. QED.

But upon exiting that medium, the photons are already going C instantaneouly. They do not accelerate up to C.

Basically any moving mass undergoes acceleration during its oscillatory motion.If light is made of of a massive particle then it would also obey the rules of gravity;hence its motion would bend in the vicinity of a large mass. It was Einstein that showed the bending of light in a gravity field.

Hence a particle of light would accelerate until it reaches it maximal speed.

Zero. This is because acceleration is the change in velocity divided by the change in time (or, more precisely, the derivative of velocity with respect to time). Light doesn't change velocity, it always moves at the speed of light.

However, there is a difference between the phase velocity of light and the group velocity of light. Please see the link below.

interesting question

well we know it has to accelerate from rest to light speed

...i have no idea. but i assume once a photon is created it instantly starts traveling at light speed.

im gonna throw this out there, maybe the instant is a Planck unit of time *shrugs*

so acceleration is = light speed / Planck time

0 m/s as it constantly travels at 3 x 10^8 m/s

if it has no acceleration it must exist outside the constraints of time otherwise it's acceleration is infinite

Light travels at the speed of 186,000 feet per second.