# The doppler effect

1. Mar 28, 2014

### Manraj singh

I was watching a video on the doppler effect by edx.org. The video said that the second wave of light given out, when the star(the light emitting object in this case) moves towards you, is not as big as the first one. So, it does not extend beyond the original wave. Why does this happen?

2. Mar 28, 2014

### UltrafastPED

We would need a reference to the video; your description is not at all clear - if that is exactly what they said, I am unable to understand what they mean.

3. Mar 28, 2014

### Manraj singh

i mean when the source is in motion, why doesn't the black circle go beyond the red one, or the green circle beyond the blue one?

4. Mar 28, 2014

### A.T.

Because they propagate at the same speed.

5. Mar 28, 2014

### dauto

Both waves are extending at the same rate and the inner one never catches up to the first because it was emitted later (just as my younger brother will never be as old as I am because he was born later and we age at the same rate.)

6. Mar 28, 2014

### Manraj singh

Oh. Thanks. But in the case of the car if the car were to accelerate, would it happen?

7. Mar 28, 2014

### dauto

If the car accelerates to a speed that is smaller than the speed of sound, The inner spheres will shift even more but remain inside the outer spheres. If the car reaches speeds higher than the speed of sound the spheres start to overlap producing a sonic boom.

8. Mar 28, 2014

### Manraj singh

That actually would be cool. Thanks

9. Mar 28, 2014

### Entanglement

I've always heard of sonic booms that happen when an object moves faster than the speed of sound but how and why exactly does it happen? Can you clarify it a little bit ??

Last edited: Mar 28, 2014
10. Mar 28, 2014

### A.T.

Nice animation here:
http://en.wikipedia.org/wiki/Sonic_boom

11. Mar 28, 2014

### Entanglement

Yeah, that's a good animation, I already realize that the object moves faster than the sound waves that it produces, but why does this cause the BOOM that we hear ?

12. Mar 28, 2014

### dauto

If a sound source is moving faster than the speed of sound the wave crests will start overlapping each other. They interfere constructively leading to a very intense but brief sound perceived as a sonic boom.

13. Mar 28, 2014

### Entanglement

Cool !! thanks, that's understood!!! Does Doppler effect happen to E.M waves ?

14. Mar 28, 2014

### davenn

yes

that how police radar ( well all radar works ) to specifically determine speed of an object

EDIT: I should also add LIDAR, Laser radar ... again, speed determination using doppler

Dave

Last edited: Mar 28, 2014
15. Mar 28, 2014

### nsaspook

16. Mar 29, 2014

### Manraj singh

Since we are on the topic can someone also explain why a cloud is formed around the object crossing the sound barrier?

17. Mar 29, 2014

### Staff: Mentor

For the electromagnetic wave equivalent of a "sonic boom", look up "Cherenkov radiation".

http://math.ucr.edu/home/baez/physics/Relativity/SpeedOfLight/cherenkov.html

18. Mar 29, 2014

### A.T.

19. Mar 29, 2014

### Manraj singh

So is this because there is so much pressure exerted on the moisture that it condenses?

20. Mar 29, 2014