The doppler effect

  1. 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. jcsd
  3. UltrafastPED

    UltrafastPED 1,916
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    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.

    So please clarify.
     
  4. ImageUploadedByPhysics Forums1396037310.464939.jpg 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?
     
  5. A.T.

    A.T. 6,282
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    Because they propagate at the same speed.
     
  6. 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.)
     
  7. Oh. Thanks. But in the case of the car if the car were to accelerate, would it happen?
     
  8. 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.
     
  9. That actually would be cool. Thanks
     
  10. 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 ?? :smile:
     
    Last edited: Mar 28, 2014
  11. A.T.

    A.T. 6,282
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    Nice animation here:
    http://en.wikipedia.org/wiki/Sonic_boom
     

  12. 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 ?
     
  13. 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.
     

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

    davenn 3,978
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    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
  16. nsaspook

    nsaspook 1,219
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  17. Since we are on the topic can someone also explain why a cloud is formed around the object crossing the sound barrier?
     
  18. jtbell

    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

    http://en.wikipedia.org/wiki/Cherenkov_radiation
     
  19. A.T.

    A.T. 6,282
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  20. So is this because there is so much pressure exerted on the moisture that it condenses?
     
  21. davenn

    davenn 3,978
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    not quite, read the first paragraph in that wiki link again

    " The condensation is caused by the sudden change in pressure across the shock waves"


    Dave
     
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