# Changing acc. of light

Fullperson
is it possible to continuously change the acceleration of light?

Homework Helper
you mean within some type of material of, say, varying density?

Troels
Fullperson said:
is it possible to continuously change the acceleration of light?

That depens on the medium.

In vacuum it does not make sense to talk about "the" acceleration of light, as for all we know it is instantenous.

indr0008
is it possible to elaborate more on this instantaneous of light's acceleration?

Fullperson
The light is in a vacuum

I'm a bit unclear; does light have an acceleration? Are we talking about propogation, like through a medium, or the actual speed of a photon? AFAIK, photons do not accelerate; theytravel at c always.

Homework Helper
The light is in a vacuum

then the velocity is always c, so the acceleration is zero.

sorry to say this, but the original question doesn't make much sense... I tried to interpret what you were saying in terms of light traveling through a medium characterized by index of refraction 'n' in which the velocity of the light is v=c/n...

so, in this case, we could suppose that there is a non-zero acceleration if the index of refraction changes with time, like in a gas that gets heated and expands (the change in density changes n).

If you are thinking of something different then you should try to explain yourself further.

Fullperson
what would happen to light if it were passed through a medium with a continuously changing index of refraction?

Homework Helper
the velocity of light would change (if the index of refraction changed) according to v=c/n.

if you call the acceleration a=dv/dt

then, in terms of the rate of change of the index of refraction dn/dt, we have
$$a=\frac{-c}{n^2}\frac{dn}{dt}$$

Phrak
then the velocity is always c, so the acceleration is zero. /QUOTE]

Hey, olgranpappy. Classically light can accelerate normal to it's trajectory in a gravitational field, but I don't think this answers the intent of the question...

In general, and in a curved spacetime light can vary in velocity with respect to a nonlocal inertial frame, I think. I don't think this was the intent of the question either.

By the way, thanks for the help with the electromagnetism question I had.

Homework Helper
By the way, thanks for the help with the electromagnetism question I had.

me? oh... no problem. you're welcome.

Homework Helper
What about light passing through a gravitational field? I recall some statements that light bends as if it were being accelerated "inwards" by the gravitational field. If this is true, than what happens if a beam of light is directed towards the center of the gravitational field, for example, aimed directly at a black hole? ... or do gravitational fields have some "magical" property that only accelerates light perpendicular to the lights direction of travle, but doesn't accelerate the light in it's direction of travel?