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Wave frequency and velocity!

  1. Sep 16, 2014 #1
    Hi! I was having some difficulty understanding this concept.

    If you know how many times per second a wave passes by a point (aka frequency) doesn't that basically say how fast it is? I understand how 2 waves, one with a short wavelength and one with a long wavelength can have the same speed but different frequencies ... but my question is why do they say that the speed will change the wavelength but not the frequency? Because, how can it change the wavelength of an already established wave... by increasing speed, doesn't the frequency automatically increase?


    Thanks!
     
  2. jcsd
  3. Sep 16, 2014 #2
    Thank you to anyone who replies!
     
  4. Sep 16, 2014 #3
    Think of a wave passing from one medium into another, and the speed of the wave changes across the boundary.

    Incident wavefronts hit the boundary at a fixed number per units time - so the number of wavefronts 'produced' in the second medium per unit time is also the same (because you need one incident wavefront to produce one in the second medium) i.e. frequency of the wavefronts will not change across the boundary.

    So how can wavelength change?

    Think of two wavefronts passing through a boundary. Say the speed of incident wavefront is 1m per second, in the second medium it is 0.5m per second. Now say the frequency is one per second (so incident wave has wavelength of 1m).

    When the first wavefront hits the boundary, it starts travelling at speed = 0.5m per second. After 1 second, the second wavefront hits the boundary. During the 1 second it took for the second wavefront to hit the boundary, the first wavefront traveled 0.5m. So the distance between the wavefronts in the second medium is now 0.5m - the wavelength has changed.

    Check out the first animation here to help you visualise this: http://www.launc.tased.edu.au/online/sciences/physics/refrac.html
     
  5. Sep 17, 2014 #4

    davenn

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    I cant believe in all the years playing with this stuff, I still cannot reconcile the statement as said on that link you gave ....

    I see the second statement as a contradiction of the first ... why doesn't it ??

    if the wavelength shortens, then the freq increases
    eg 100 MHz = 3 metres wavelength 1000MHz (1GHz) = 0.3 metre
    ( shorter wavelength higher freq)

    Dave
     
  6. Sep 17, 2014 #5
    This is only valid if the speed is constant. The speed in the first statement is not constant.
     
  7. Sep 17, 2014 #6

    davenn

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    hi willem

    so if you slow the velocity (speed) of the wave, the freq will increase proportionally to the drop in velocity ?

    Dave
     
  8. Sep 17, 2014 #7

    CWatters

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    If 100 cars an hour drive under a bridge the frequency is 100 cars an hour regardless of how fast they are driving.

    If they speed up after passing the bridge the gap between cars increases but the frequency is still 100 per hour at the next bridge.

    The question appears to be how do the wave fronts speed up so the gap between them increases. That depends on what sort of wave and the medium its moving through.
     
  9. Sep 17, 2014 #8

    PeroK

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    And the alternative situation is where the cars are all joined together (like wagons on a train). As the train speeds up the "wavelength" (distance between the wagons) remains the same but the frequency (how many wagons pass under the bridge) increases.

    In short, it depends whether the wave retains its shape or stretches out as it speeds up.
     
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