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Homework Help: Counting the number of waves

  1. May 17, 2012 #1
    1. The problem statement, all variables and given/known data

    When you shine a light on the ripples in a tank you get dark and bright patches like this:

    http://img687.imageshack.us/img687/4343/rippletank.jpg [Broken]

    So my question is: how do we count the number of waves?

    In my experiment I am trying to find the wavelength by dividing the distance travelled by a train of waves by the number of waves in it. Do we have to count the number of bright fringes or the shadows?

    3. The attempt at a solution

    We are not trying to find the distance from one trough/crest to the next (that would be the wavelength). So, what is "one wave" in this case? How many waves are there in the picture I've posted (I can see 7 bright patches and 7 shadows)? :confused:
    Last edited by a moderator: May 6, 2017
  2. jcsd
  3. May 17, 2012 #2
    Well it's not as simple as just counting bright or dark fringes.

    Because an entire wavelength includes both, so in your picture you actually have a fractional amount of wavelengths.

    Unless you want to estimate that there are 6.9 wavelengths or something like that then the best thing to do would be to adjust your experiment such that you are measuring a region that you are fairly sure contains an integer number of wavelengths.

    But 1 wavelength which is what I would say counts as having "one wave" extends from crest to crest, or trough to trough.
    If I start at the first bright spot on the left and go to the next crest that is 1.
    So I see 6 full wavelengths in the image.
    Then there is a very tiny little sliver on the very left, and also about half a wave on the right side.
    So idk... 6.7 waves?

    Also, I think you should avoid calling them "waves", and use "wavelengths" instead. That entire thing is one wave, which spans several wavelengths. (of course this last sentence is just semantics, I could be wrong, but that's how I think it goes)
  4. May 17, 2012 #3
    Thanks for the input. They call them "waves" in the pamphlet that describes the experiment. The aim is to find the wavelength by using a ruler to measure the length of as many waves as possible, and then dividing them by the number of waves.

    So "one wave" would be just one wavelenght? So, the start of a bright patch to the end of a dark patch would count as one wave?
  5. May 19, 2012 #4
    Sorry I didn't notice you responded sooner, for some reason it didn't notify me.

    But "one wave" should be measured from bright spot to bright spot.

    Or if you prefer, you can go from dark spot to dark spot. That might seem more intuitive since you are including the entire bright region.
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