# speed of light

## Recommended Posts

Does the "speed of light" change.

I just had a thought .I know that sound waves go faster when going through denser objects!! does the speed of light change when going through denser objects??? If yes, what speed do we clasify as 'speed of light'(not the actual speed)when light travels through a vacume????

thnx guys!!

##### Share on other sites

The speed of light © does not change, this is the speed of individual photons, and the speed of a light pulse in a vacuum. The speed of a light pulse is reduced when it passes through material though, slightly in air, quite allot more in glass. It depends on the refractive index of a material, I'd recommend reading wikipedia about this and then asking any specific questions from the article here

http://en.wikipedia.org/wiki/Refractive_index

##### Share on other sites

You also need to remember that light isn't transmitted like sound is, by molecules and atoms bumping together and vibrating. It's transmitted by moving photons. When photons hit atoms, they're absorbed and then re-emitted a split second later, so denser materials make light slower. (Just remember that the photons themselves always move at the speed of light, c -- the light is slower because of the delay between being absorbed and re-emitted.)

##### Share on other sites

When you have particles going through a material at a speed faster than the speed of light in that material, then they emit Cherenkov radiation. It's like the electromagnetic version of a sonic boom.

##### Share on other sites

I thought the boom comes from passing from v(plane)<v(sound) to v(plane)>v(sound), not from being in the later state?

##### Share on other sites

I thought the boom comes from passing from v(plane)<v(sound) to v(plane)>v(sound), not from being in the later state?

No, the shock wave that forms the "boom" is continually produced as long as the craft is supersonic. You hear the boom when the edge of the shock wave passes you.

##### Share on other sites

When you have particles going through a material at a speed faster than the speed of light in that material, then they emit Cherenkov radiation. It's like the electromagnetic version of a sonic boom.

Escuse my complete ignorance here please guys but I have a couple of questions. 1) I thought things couldn't go faster than light?

2) If a space ship flying past an outside stationary observer in space was going at ...lets say 0.5C and turned on the headlights. What is the relative speed of the light beam from the head lights compared to A) the Space ship and B) the stationary observer?

I could probably look this up but from some of the reading I have done I am getting a bit confused - It was years ago when I done this. Thanks.

##### Share on other sites

1. Nothing can go faster than the speed of light in a vacuum. It's the speed of light in that particular medium that can be broken, causing Cerenkov radiation.

2. From the observer's frame, the light's traveling at c and the spaceship at 0.5c. From the spaceship's frame, the light is also traveling at c and the spaceship is stationary.

##### Share on other sites

1. Nothing can go faster than the speed of light in a vacuum. It's the speed of light in that particular medium that can be broken, causing Cerenkov radiation.

2. From the observer's frame, the light's traveling at c and the spaceship at 0.5c. From the spaceship's frame, the light is also traveling at c and the spaceship is stationary.

Thanks - that's exactly what I thought.

##### Share on other sites

$u' = {v+u \over 1+(v/c)(u/c)}$

Where v is the velocity of frame B compared to frame A, in frame B something is moving with velocity u, which is measured in frame A to be u'

If you put c as u into that you get c out as u'.

##### Share on other sites

$u' = {v+u \over 1+(v/c)(u/c)}$

Where v is the velocity of frame B compared to frame A, in frame B something is moving with velocity u, which is measured in frame A to be u'

If you put c as u into that you get c out as u'.

It's where time dilation comes into it yes? The space ship actually moves slower in the stationary observer's reference frame? Which is why if the observer and the ship's captain are twins, one will age faster than the other because his time has passed differently for him.

##### Share on other sites

No the space ship's velocity of 0.5c is measured in the observers frame...

This is a relativistic effect linked closely to time dilation and length contraction...

##### Share on other sites

Well Thnx it makes more sence now!!!

##### Share on other sites

• 3 weeks later...

I'm new, so please wait 30 seconds before you fall on the floor laughing. That was please with a p. I'm kind of refering to Dr. Evil's thoughts. I have been thinking lately about the relative speed of light. It's constant. This may be a thought. No matter how fast light source can move, when the light is emitted it is as if an instant picture (A photo finish) was taken and only showed the distance between obserber and light source. A picture doesn't show v. Now how to tackle the irf of the camera.

Now you can laugh.

##### Share on other sites

Does the "speed of light" change.

I just had a thought .I know that sound waves go faster when going through denser objects!! does the speed of light change when going through denser objects??? If yes, what speed do we clasify as 'speed of light'(not the actual speed)when light travels through a vacume????

thnx guys!!

Yes in that the average speed through a medium is less than that in a vacuum due to the process of absorption and reemission. The quantity labeled c is called the speed of light in a vacuum and which applies locally in a gravitational field too.

Pete

## Create an account

Register a new account