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B Doppler Effect -- How fast would you need to go for a red light to appear green?

  1. Jun 21, 2017 #1
    I was driving the other day, and I was about to go past a green light when it turned yellow, so I sped up. I got past the light but but it got me wondering, how fast would you need to go for a red light to appear green due to the Doppler Effect.
    (Excuse me if this question doesn't belong in general physics, I wasn't sure where it belonged)
  2. jcsd
  3. Jun 21, 2017 #2
    Do you know the formula for the Doppler effect and what frequencies correspond to red and green?
  4. Jun 21, 2017 #3
  5. Jun 21, 2017 #4
    I do know that the Doppler effect causes light or sound waves to have smaller or thigher bends in front of them when you go faster. Green light wavelength is tighter than red, so this is possible (theoretically that is, I know that this is going to be fast and a car would probably burn up before it reached that speed). People in rockets going to space experience this.
  6. Jun 21, 2017 #5


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    OK you need to find the wavelength of green and red light and then use those formula

    here's a head start for you....

    convert wavelength to frequency
    wavelength of red light is around 633nm
    frequency = speed of light / wavelength
    frequency = 3.0e8 m/s / 633nm
    frequency =

    you have a go at the rest

  7. Jun 22, 2017 #6
    Dude, I have no degrees whatsoever, I'm asking because I don't know, I want to know, but I can't do this equation.
  8. Jun 22, 2017 #7
    Urban Legend holds that, back in the '60s on the West Coast, a couple of young physicists tried this ploy to side-step certain driving bans for 'running the red'. Taking advice, the local judge stuck them with fixed-fee speeding fines for each additional 10 mph over the posted limit. After hasty calculation, the perps pleaded to the original charge...
  9. Jun 22, 2017 #8


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    If one cannot do the equations then the obvious point to learning the result is to have a big number to go "woo woo" over. If the supply of such numbers is running low, there are always sports science shows to watch.

    Rough rule of thumb (classical Doppler): If you want to halve the frequency, you can run away from the source at half the speed of the wave. If you want to double the frequency you can run toward the source at the speed of the wave.
  10. Jun 22, 2017 #9


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    well you shouldn't have marked your thread as I for intermediate then, aye !
    that assumes you have a very good education
    [Mentor's note: the thread level has been changed to "B"]

    you didn't even attempt to finish off the example I gave above and then do the same for the green light
    Last edited by a moderator: Jun 22, 2017
  11. Jun 22, 2017 #10


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    Don't sell yourself short. You can do it.
    And the reason we aren't spoon-feeding you the answer is that we know you can do it, and we know this because we've all been where you are now.
  12. Jun 23, 2017 #11
    I did attempt to, actually. And I failed. I thought the thread level meant how hard the question was.
  13. Jun 23, 2017 #12


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    Show us your working and someone will see where you went wrong.
  14. Jun 23, 2017 #13
    This post was mostly ignored, so I'm quoting it. Even without knowledge of algebra, with this rule of thumb the OP should get into the ballpark of speed necessary. OP, look up the speed of light, and the frequencies of green and red, and you have all the information needed.
  15. Jun 27, 2017 #14
    To my calculations, the speed at witch you would must travel to run a red light claiming it was green due to the Doppler affect you would have to go
    111,600,000 mph
    By the way, I have no degrees because I am 12
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