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Homework Help: 2 questions about waves, help!

  1. Mar 19, 2009 #1
    Sources A and B emit long-range radio waves of wavelength 320 m, with the phase of the emission from A ahead of that from source B by 90°. The distance rA from A to a detector is greater than the corresponding distance rB from B by 82 m. What is the magnitude of the phase difference at the detector?

    In a double-slit experiment the distance between slits is 5.2 mm and the slits are 0.82 m from the screen. Two interference patterns can be seen on the screen: one due to light of wavelength 450 nm, and the other due to light of wavelength 580 nm. What is the separation in meters on the screen between the m = 4 bright fringes of the two interference patterns?

    The first one is actually an online-test question. I have tried 3 times using the method from the textbook, but i still haven't got the right answer. Can anyone help me? THANKS!
  2. jcsd
  3. Mar 19, 2009 #2


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    You understand, don't you, that you have to show what you have done yourself?
  4. Mar 19, 2009 #3
    Hello! What i have done for the first question is:
    devide the wave length 320m by 4, since A is 90° ahead (a quarter of a cycle)
    then I take 82m away from it, since the wave from source A goes 82m further
    finally i just take the ratio of the phase difference with the wavelength and multiply the result by 2π in order to convert the unit to radian.

    is it correct?
  5. Mar 19, 2009 #4
    i can't do the second one. i thought the 2 lights will not have any bright fringes, because the wavelengths are different, double-slit interference will not occur...
    do you think so?
  6. Mar 19, 2009 #5
    1. How much is A in front of B? (90 deg)
    How big is the phase difference between A and B just because wave A has to travel longer?

    2. For both 450 nm and 580 nm a double-slit interference will occur. What distance for m=4 at 450 nm and what distance for the 580 nm wave?
  7. Mar 20, 2009 #6
    I've got the answers now! thanks!!
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