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Homework Help: Need help with a diffractiongrating problem.

  1. Jan 22, 2009 #1
    1. The problem statement, all variables and given/known data
    Light with a wavelength of 400nm passes through a 1.00x10^4 lines/cm diffraction grating. What is the second order angle of diffraction?

    a) 21.3 c)56.5
    b) 53.1 d) 72.1

    2. Relevant equations
    d*sin(theta)=m* wavelength


    3. The attempt at a solution

    I rearranged the equation to: Theta= sin^-1({wavelength*order}/ d)
    Theta= sin^-1 (4*10^-7*2/{1/1*10^2)= sin^-1(8*10^-9)= 4.58366

    Obviously, this isnt the answer. Could someone show me how to figure these problems out? I have a test coming up and i am completely stumped on all of these types of questions.
     
  2. jcsd
  3. Jan 22, 2009 #2

    rl.bhat

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    Theta= sin^-1 (4*10^-7*2/{1/1*10^2)= sin^-1(8*10^-9)= 4.58366

    Here d = 0.01/1x10^4 m or 10^-6 m.
     
  4. Jan 23, 2009 #3
    Ok i reworked the problem, and got a different answer, but it is still no where near any of the multiple choice answers! what am I doing wrong?
     
  5. Jan 23, 2009 #4

    Redbelly98

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    Can you show your second calculation, with the new value of d? Otherwise we can't see what's wrong.

    EDIT:

    There is an error here, besides the incorrect value of d. Do you see it?
     
  6. Jan 23, 2009 #5
    Holly cow! I just re-reworked the problem for about the hundredth time and got the answer of 53.1, which is actually one of my choices! and I should imagine that the (8*10^-9) is wrong. The answer should be 8*10^-1. Right? or am I STILL messing up?
     
  7. Jan 23, 2009 #6

    Redbelly98

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    Yes, it is 0.8 using the correct value of d=10-6m.

    Or, if d=(1/102)m or 10-2m as you had earlier, you should have gotten 8x10-5 for the expression.
     
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