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Homework Help: At what distance x do the waves have a phase difference of ___?

  1. Apr 18, 2015 #1
    1. The figure shows two point sources S1 and S2 that emit sound of wavelength λ = 1.8 m. The emissions are isotropic and in phase, and the separation between the sources is d = 18.0 m. At any point P on the x axis, the wave from S1 and the wave from S2 interfere. Start with P very far away (x = infin). As you then move P in along the x axis toward the origin, (a) does the phase difference between the waves increase or decrease? At what distance x do the waves have a phase difference of (b) 0.50λ, (c) 1.00λ, and (d ) 1.50λ?
    The diagram: (d is the distance from S1 to S2)
    S1---------P-----> x

    2. Fully Destructive interference phi = (2m+1)pi, Fully constructive interference phi=m(2pi), deltaL=sqrt(324+x^2)-x

    3. The attempt at a solution
    Using the above equation of deltaL=sqrt(324+x^2)-x, I obtained the answers 161.5m, 80m, and 52.5 meters. sqrt(324+x^2) - x= 1 ,sqrt(324+x^2)-x=2, and sqrt(324+x^2)-x= 3

    I'm confused as to why my answers are incorrect, I thought when the phase constants were 0.5lambda,1lambda, and 1.5lambda, the distance associated with these were 1,2, and 3. I know that my equation is correct from pythag, but im not 100% sure about my x value. Any assistance is greatly appreciated. Thanks in advance.
    Last edited by a moderator: Apr 19, 2015
  2. jcsd
  3. Apr 18, 2015 #2


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    Staff: Mentor

    The formulas look right, but the x-values you got as solutions are not.
    Did you use a wave-length of 2 meters? It is just 1.8 m.
  4. Apr 18, 2015 #3
    I was looking at an example which had the wave-length as 2 meters, I must have gotten confused. Can you point me in the right direction for solving it with the 1.8m wave-length? I'm honestly confused on what to do with the wavelength.
  5. Apr 18, 2015 #4
    Thank you, I think I understand what I did wrong, hopefully my answers come out correct, will update after trying my hypothesis. :)
  6. Apr 18, 2015 #5
    All good to go, if you dont mind helping me with another problem or do I have to create another thread?

    In the figure, a sound of wavelength 0.600 m is emitted isotropically by point source S. Sound ray 1 extends directly to detector D, at distance L = 10.5 m. Sound ray 2 extends to D via a reflection (effectively a "bouncing") of the sound at a flat surface. The reflection occurs on a perpendicular bisector to the SD line, at distance d from the line. Assume that the reflection shifts the sound wave by 0.500λ. For what least value of d (other than zero) do the direct sound and the reflected sound arrive at D (a) exactly out of phase and (b) exactly in phase?

    Relevant equations: the afore mentioned equations

    Attempt: I'm honestly not sure where to even start, if you could point me in the correct direction, please and thanks in advance.
    Last edited: Apr 18, 2015
  7. Apr 18, 2015 #6
    So first try thinking about exactly what out of phase and in phase mean. Then, you should be able to calculate how much of a wavelength is required for each condition and use triangles find what ##d## has to be. You know how far it has to go, written in terms of ##d##, just solve for ##d##.

    So for example:
    Ray 2 has to travel ##\sqrt{{\frac{L}{2}}^2 + d^2}##, then it gets shifted by ##0.5λ##, then it has to travel the same distance again.

    That help?
  8. Apr 19, 2015 #7
    Yes, thank you so very much!
  9. Apr 19, 2015 #8


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    Staff: Mentor

    FYI, it usually works best if you create a new thread for each new question that you want help with. :smile:
  10. Apr 19, 2015 #9
    Thanks, as you can tell,I am a "starter" :smile:
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