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Homework Help: Diffraction of light question - How do I approach this?

  1. Jun 12, 2012 #1
    1. The problem statement, all variables and given/known data
    Blue light of wavelength 485.6nm from a star is incident normally on a diffraction grating. The light is diffracted into a number of beams, as shown in Fig 5.4.(attached)

    The angular separation of the two second-order beams is 45.72 degrees.
    Calculate the number of lines per millimetre on the grating.

    2. Relevant equations

    3. The attempt at a solution

    I genuinely do not know where to start the question. I know the formula for Young's double slit experiment however this does not seem to relate to it at all. Any suggestions for starting the question would be really appreciated.

    Attached Files:

  2. jcsd
  3. Jun 12, 2012 #2


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    Read your textbook. It surely mentions diffraction gratings.
  4. Jun 13, 2012 #3
    Never seen the formula before. The question is from an old past paper so it might be off the specification. The only thing relating to path difference that I have seen is the Young's modulus experiment. Anyways I had an attempt, is my working correct?

    Θ = 45.72/2 = 22.86 degrees
    2*485.6*10^-9 = d sin(22.86)
    d = 2.50*10^-6 m
    d = 2.50*10^-3 mm
    N = 1/d = 1/(2.50*10^-3) = 400
    Lines per mm = 400

    There is no mark scheme for this so if someone could verify my working that would be great.
  5. Jun 13, 2012 #4


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    Looks good. With a diffraction grating the only difference is the maxima become sharper otherwise it's the same as the two-slit experiment
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