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Running red lights

  1. Jul 16, 2013 #1
    1. The problem statement, all variables and given/known data

    A police officer pulls you over for running a red light. You tell the
    officer, “But the light looked green to me!” How fast would you have to be traveling for
    this statement to be true?

    2. Relevant equations

    ???

    3. The attempt at a solution

    Can anyone point me in the right direction?
     
  2. jcsd
  3. Jul 16, 2013 #2

    Dick

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    It's a doppler effect question. Look up the approximate wavelengths of red and green light and figure out how fast you would have to be travelling to shift one into the other. It won't be an exact number, since 'red' and 'green' aren't exact wavelengths.
     
  4. Jul 17, 2013 #3

    CWatters

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  5. Jul 17, 2013 #4
    λred=650nm=650*10^-9m
    λgreen=510nm=510*10^-9m

    f*λ=c c=2.99*10^8

    fo=fs(1+/-vrel/c)

    fo=5.863*10^14
    fs=4.6*10^14

    5.863*10^14Hz=4.6*10^14Hz (1-vrel/c)

    vrel=3.8%*10^8m/s

    Is this correct?
     
  6. Jul 17, 2013 #5

    rude man

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    Wrong formula. needs correction for relativistic effect.

    I don't know what the % sign is doing in your answer, but if it shouldn't be there then you'd be going faster than the speed of light, which Al said is a no-no.
     
  7. Jul 17, 2013 #6

    PeterO

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    My understanding is that "red-shift" occurs when the light source is moving away from you (relatively speaking), but the motorist is clearly moving towards the light.
     
  8. Jul 17, 2013 #7

    OmCheeto

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    Quick! You still have time to delete your dyslexic post!

    ps. I agreed with you at first, as I am totally dyslexic. But then I saw that Dick and CWatters had commented, and would have caught that in a second, so I double checked, and red light is the lower frequency, meaning lower energy. Ergo, going faster would shift the light to a higher frequency, and therefore to green.
     
  9. Jul 17, 2013 #8

    PeterO

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    Correct - damn it, I was trying to make the Green light appear Red !!
     
  10. Jul 17, 2013 #9

    haruspex

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    As rude man says, you'd do better using the relativistic formula: http://en.wikipedia.org/wiki/Relativistic_Doppler_effect.
    But with the formula you used you should have been fairly close. I don't understand what you did after 5.863*10^14Hz=4.6*10^14Hz (1-vrel/c). You should have got 27%*c. The relativistic version makes it a bit lower.
     
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