# Dopplers law in light

1. May 13, 2007

### bassplayer142

I'm sure that the doppler law would come into effect in light as it does in sound. Not the changing of light color but breaking the light barrier as I'll call it. I'm just wondering if we ever see it or if we never see it in life. Even when you turn on a light shouldn't you be seeing this light barrier as a flash of light because it is going the speed of light or not.

2. May 14, 2007

### lalbatros

Breaking the light barrier in vacuum is impossible, since nothing (physical) can go faster than light.
But within a material it is possible to break the "light barrier".
Within a material, light can go slower that in vacuum.
Therefore a fast energetic particle can go faster than the speed of light in the given material.
This procudes some light emission called "Cerenkov radiation".

You could observe it as a faint blue light in nuclear fuel storage pools.

Last edited by a moderator: May 2, 2017
3. May 14, 2007

### bassplayer142

So in order to break the light barrier you can't just go the speed of light but faster.

4. May 14, 2007

### Staff: Mentor

I'm not sure what analogy you are trying to draw here, but do you hear a blast of sound when someone starts talking? Or are you saying that since light goes the speed of light, it should form some shockwave? Does sound do that?

There is a regular doppler effect for light that changes it's color, but I'm not sure how that relates to your question - it doesn't make much sense to me.

5. May 14, 2007

### bassplayer142

Yes, when you hit the speed of sound there is a shockwave. If you went faster then light wouldn't it be a light wave. A huge burst of light that goes off from the object.

6. May 14, 2007

### Staff: Mentor

It's the light source that must be traveling faster than light for the kind of "shock wave" effect I think you are talking about (and which lalbatros explained). Just turning on a flashlight does nothing.

7. May 14, 2007

### Staff: Mentor

Right - turning on a flashlight doesn't produce a burst of light just like talking doesn't produce a burst of sound.

8. May 14, 2007

### Claude Bile

Bassplayer; The Cerenkov radiation lalbatros mentioned is the optical analogy of a sonic boom (which I'm guessing is what you refer to when you say "shock wave").

Claude.