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Dispersion by diffraction grating

  1. Nov 25, 2009 #1
    1. The problem statement, all variables and given/known data

    White light containing wavelengths from 396 nm to 756 nm falls on a grating with 7300 lines/cm. How wide is the first-order spectrum on a screen 2.15 m away?

    2. Relevant equations
    y=m*L*wavelenth/(d)


    3. The attempt at a solution
    using the equation I have found y1 to be 62.149 and y2 118.635. I took the difference of the two which gave me 56.486 cm, but that is wrong for some reason. Please help.
     
  2. jcsd
  3. Nov 25, 2009 #2
    Are you sure of that that's the equation for diffraction? Checking the units, it's quite inconsistent... Check the equation again...
     
  4. Nov 25, 2009 #3
    That's the equation I'm used to. If we assume that the angle is small then sintheta=y/L
    dsintheta=m*wavelength
    d(y/L)=m*wavelength
    m*wavelength*L/(d)=y ? I don't know what's wrong with the equation. For the units I coverted everything to cm because 1/7300=d in cm.
     
  5. Nov 25, 2009 #4
    Please help, I only have a couple of hours left.
     
  6. Nov 25, 2009 #5
    I got 56.502 cm, using the equation. we seem to get the same answer.
     
    Last edited: Nov 25, 2009
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