# Changing acc. of light

1. Apr 18, 2008

### Fullperson

is it possible to continuously change the acceleration of light?

2. Apr 19, 2008

### olgranpappy

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

3. Apr 19, 2008

### Troels

That depens on the medium.

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

4. Apr 19, 2008

### indr0008

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

5. Apr 19, 2008

### Fullperson

The light is in a vacuum

6. Apr 19, 2008

### LURCH

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.

7. Apr 19, 2008

### olgranpappy

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.

8. Apr 19, 2008

### Fullperson

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

9. Apr 19, 2008

### olgranpappy

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
$$a=\frac{-c}{n^2}\frac{dn}{dt}$$

10. Apr 19, 2008

### Phrak

11. Apr 19, 2008

### olgranpappy

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

12. Apr 20, 2008

### rcgldr

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?