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Homework Help: Diffraction grating experiment

  1. Nov 20, 2013 #1
    I did an experiment where a red laser went through a diffraction grating
    The distance between the diffraction grating to the screen is 10cm (x)
    Later I measure the distance from the central dot to the first dot (4.5cm) (y)
    after this I measure the distance from the central dot to the second dot(13.1cm) (y)

    I used this formula to calculate the wavelength of the red laser: d*tan(y/x) = m λ
    where m = 0,1,2... and d = 1.67*10^-6 in this experiment.

    The problem which I've got is that I got two different wavelengths for the same red laser
    692.4nm and 765.6nm.
    My question is this, should I add these two wavelengths and then divide it by two to get the "average" wavelength or should I just say in the experiment that the wavelength "765.6nm" is a random error; thus, we should not take it to the consideration?
  2. jcsd
  3. Nov 20, 2013 #2
    any one please, if this is a stupid question then please tell me
  4. Nov 20, 2013 #3


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    Homework Helper

    The equation you quoted is wrong. The correct equation is d sinθ = mλ.

    What you did was to replace sinθ by tanθ=y/x, so you calculated the wavelength from the formula d y/x = mλ. But sinθ can be approximated by tanθ only for small angles, that is, when y<<x. That was not valid in your experiment. Find θ=arctan(y/x), and use the formula d sinθ = mλ to get the wavelength.

  5. Nov 20, 2013 #4
    thank you so much
  6. Nov 20, 2013 #5


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    You are welcome.:smile:

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