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Homework Help: Light problem -- diffraction grating distance between adjacent bright fringes

  1. May 1, 2015 #1
    1. The problem statement, all variables and given/known data
    Light from a laser with a wavelength of 760 nm is directed at a diffraction grating of 1500 lines/cm. If the diffraction grating is located 1.5 m from the screen, calculate the distance between adjacent bright fringes.

    2. Relevant equations
    Δx = Lλ / d
    760 nm = 7.60 x 10^-7 m = λ
    1.5 = L
    1cm/1500 = 6.67 x 10^-6 m = d

    3. The attempt at a solution
    Δx = (1.5 m)(7.60 x 10^-7 m) / (6.67x 10^-6 m) = 1.71 x 10^-13 m

    I'm not sure if this is correct please help.
  2. jcsd
  3. May 1, 2015 #2


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    Can it be correct? You see the bright fringes on the screen. Their distances is in the range of mm or cm, Can 10-13 m true???
  4. May 1, 2015 #3
    I saw other people getting 1.72 cm which is 0.0172 m but the the answered I wrote is what I got in the calculator so I'm not sure how they got that
  5. May 1, 2015 #4


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    You typed in wrong number in the calculator when you divided by 6.67x10-7.

    I guess you did not use parentheses.
    Last edited: May 2, 2015
  6. May 2, 2015 #5


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    Dear sudent,

    you want to avoid this kind of disaster by estimating the outcome of a calculation before using an electronic calculator. In this case you divide 7.6 by 6.7 which is a little more than 1, and 10-7 by 10-6 which is 10-1.
    Times 1.5 should give a little over 0.15. So your answer is 17.1 cm.
  7. May 2, 2015 #6
    actually Ehild was right I did not use parenthesis when I divided by 6.67x10-7 so the actual answer is 0.171 m.

    Thank you for your help guys!
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