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Homework Help: Diffraction Grating?

  1. Nov 16, 2008 #1
    Diffraction Grating??? [URGENT]

    1. The problem statement, all variables and given/known data
    Consider two monochromatic sources of light, blue light of wavelength λb = 460 nm and red light of wavelength 670 nm. The red light is shun upon a diffraction grating of density, nr = 10,000 lines per cm. A diffraction grating with what line density (lines/cm) is required to have the 2nd maximum (m=2) of blue light coincide with the 1stmaximum (m=1) of red light?

    2. Relevant equations
    .....I don't really know.

    3. The attempt at a solution
    I really don't have a clue, i don't even know relavant equations!

    Please help. THanks
  2. jcsd
  3. Nov 16, 2008 #2


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    Re: Diffraction Grating??? [URGENT]

    Have you read your course text or class notes?
  4. Nov 16, 2008 #3
    Re: Diffraction Grating??? [URGENT]

    Standard diffraction grating formula:

    [tex]a(sin\theta_i + sin\theta_m) = m\lambda[/tex]
    [tex]a = \frac{1}{N}[/tex]

    theta_i and theta_m are incident and m'th diffracted angles respectively. (Though you don't need them).
    Form two equations and solve for N.

    I'll leave you to it. I'm off to bed :biggrin:
  5. Nov 17, 2008 #4
    Re: Diffraction Grating??? [URGENT]

    I'm confused, what are theta then? And how should I fit all values of the red and blue light in this formula?
  6. Nov 17, 2008 #5
    Re: Diffraction Grating??? [URGENT]

    Don't overthink this question - it is simpler than you think.
    You're told that m=2 for one wavelength must lie on m=1 for another wavelength. (Therefore theta_m will be the same for both wavelengths).
    (Note also the incident angles are the same because you assume the sources are collinear).

    Form 2 diffraction grating equations given the known variables. Solve for the missing N.
  7. Nov 17, 2008 #6
    Re: Diffraction Grating??? [URGENT]

    I got 7282.6, if it's right....
  8. Nov 17, 2008 #7
    Re: Diffraction Grating??? [URGENT]

    It's what I got too.:smile:
  9. Nov 17, 2008 #8
    Re: Diffraction Grating??? [URGENT]

    Okay Thanks!
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