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Homework Help: Water wave

  1. Apr 30, 2005 #1
    This question is related to the wave nature of light from Giancoli:

    Water waves having parallel crests 2.5cm apart pass through two openings 5cm apart in a board. At a point 2.0m beyond the board, at what angle relative to the "straight-through" direction would there be little or no wave action?

    The reason I am having a problem with this question is because I cannot visualize what a "parallel crest watever wave" is. Is it like the one in this picture (http://hyperphysics.phy-astr.gsu.edu/hbase/phyopt/imgpho/sinslitwid.gif) ?
  2. jcsd
  3. Apr 30, 2005 #2
    Yes, those are paralllel crests.
  4. Apr 30, 2005 #3
    how can i find the wavelength of the water wave?
    Is it simply just 1/2.5cm or 40m?
    Last edited: Apr 30, 2005
  5. Apr 30, 2005 #4
    what's the definition of wavelength?
  6. Apr 30, 2005 #5
    distance from crust to crust, gottcha
  7. Apr 30, 2005 #6
    I find the angle to be 14.48 degree, but I didn't use the 2 meters that the question provdied. Am I missing something?

    I use [tex] d sin (\theta) = (m + \frac {1}{2} )(0.025m)[/tex] where d = 0.05m, and m = 0 to find [tex]\theta[/tex].
    Last edited: Apr 30, 2005
  8. May 1, 2005 #7


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    Since the problem asked you for the angle, and not a distance from the midpoint, you do not need the 2m as long as that distance is much greater than a wavelength. If the distance were only a couple of wavelengths, the approximations used to derive the equation you used would not be justified. Look carefully at this diagram and you will see that there are two angles [itex]\theta[/itex] and [itex]\theta^\prime[/itex] that are only approximately equal.

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