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No of red fringe will be seen

  1. Sep 12, 2014 #1
    1. The problem statement, all variables and given/known data

    In a Young's double slit experiment , blue monochromatic light of wavelength 450nm is used to illuminate the slits which are 0.60mm apart. A screen is placed 1.5m form the slit is used to catch the interfrence pattern .
    If red light is used (700nm) , how many red light will be seen? the ans is 7

    2. Relevant equations

    3. The attempt at a solution
    dsinθ = m λ
    0.6x10^-3 ( sin 90 ) = m (700x10^-9)
    m = 857
    no fringe can be seen = 857+ 857+1 ..
    What's wrong with my working?
  2. jcsd
  3. Sep 12, 2014 #2
    where does sin(90) come from?
  4. Sep 13, 2014 #3
    Sin90 beacuse I wamt to calculate themaximum order of fringes. Is my concept wrong?
  5. Sep 13, 2014 #4


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    Staff: Mentor

    I would agree with you. Perhaps the question is meant to say something like "a 1 metre square screen is placed 1.5m from the slits".

    Can you work backwards, and work out what screen dimension would show just 7 bands either side of the central maximum?
  6. Sep 13, 2014 #5
    Do you meam the fringe is only formed on 1mm square of screen?
  7. Sep 13, 2014 #6


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    Staff: Mentor

    If you were wanting to see just the first order fringe, yes, that might be about right. But didn't you say the answer is n=7?

    Could you check the question, and make sure you haven't got some detail wrong.
    Last edited: Sep 13, 2014
  8. Sep 13, 2014 #7
    How is the part about blue light relevant? It seems that the question is incomplete or mixed-up somehow.
  9. Sep 13, 2014 #8
    that's the whole question i have typed above...
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