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Superposition, amplitudes and superposed waves.

  1. Apr 8, 2008 #1
    First off:
    Sorry. I'm a Norwegian girl, so my English may not be of very good quality.
    Now that's said...

    I have this problem to solve.
    Only thing is - I don't have a clue how!
    I'm no good at physics, but unfortunately I have to have this class.

    My problem is this:

    Task 1a)
    Two harmonic waves are mutually coherent.
    One wave has an intensity of 4 unit^2,
    the other wave has the intensity of 9 unit ^2,
    their superposition wave will have an amplitude of

    Task 1b)
    Two waves are mutually incoherent.
    One wave has an amplitude of 3 unit,
    the other wave has an amplitude of 4 unit.
    The intensity for the superposed wave should be?

    How do I calculate this?
    What formula am I suppose to use?
    And if you only wan't to give me the formula,
    can you please explain what the different letters, symbols and stuff means (so that I can solve it myself, if you don't want to give me the direct answer?)
  2. jcsd
  3. Apr 8, 2008 #2

    1a)the intensity is propotional to the amplitude squared. if they're mutually coherent you add the amplitudes and square the resultant (ans= 25 unit^2 i think)

    1b)if mutually incoherent, there's a phase difference. so for all you know, a peak of one wave could coincide with a trough of the other. The resultant amplitude is anywhere from 1 to 7 (ie. intensity=1 to 49)
  4. Apr 8, 2008 #3
    Thank you for your reply, but did you perhaps overlook that it said "9 unit^2"?

    If I now do this your way it'll be like this rigth?
    9^2 + 4^2 = 97
    And you believed I should square the result so that the final answer would be 97^2.

    I don't know if this is right,
    but either way I'm really thankful for your advice/opinion.
    It's certainly worth a try!

  5. Apr 8, 2008 #4
    The second answer you gave me seems to be wrong,
    I found out to crack it on my own.
    Or... At least i think I did ;)

    Because the waves are incoherant I have to use the formula:
    I = I1 + I2
    = 3^2 + 4^2
    = 25

    The intensity for the superposed wave should be 25.
  6. Apr 8, 2008 #5
    ^^ Not sure if that's correct TBH.

    I(total) is infact I1+I2+(A1*A2)cos(X)

    where X is the phase difference between the two waves and A is the amplitude. Incoherent waves, by definition, have a difference in phase.
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