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Homework Help: Question About Diffraction Orders

  1. Jan 8, 2017 #1
    1. The problem statement, all variables and given/known data
    Light is normally incident on a double slit system. I'm asked to calculate the number of "orders(maxima)" visible on a screen placed in front of the slits. The light wavelength and slit separation are given. I understand that I can calculate the "order" from m=d/λ but does this number tell me the total maxima? Or am I to understand that there is a central maximum at m= 0 and then maxima at every ±m? For instance I got m=8.4; Does that mean I have 17 visible maxima or 8?


    2. Relevant equations

    3. The attempt at a solution
    I have calculated m but I don't quite understand it's meaning.
  2. jcsd
  3. Jan 8, 2017 #2

    Dr Transport

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    Science Advisor
    Gold Member

    There are two things that need to be considered here, the grating equation which you quote gives the angular distribution of the maxima from a single slit. Since you have a pair of slits, there is more going on here. The diffraction from a single slit gives an intensity [itex] \approx sinc^2()[/itex] envelop, the addition of the second slit modulated the intensity within this envelop.

  4. Jan 8, 2017 #3
    Okay, thanks for the input, I'm going off of Halliday and Resnick Ch 36.7. He says that the d is the spacing between the slits and I assume that if it's far enough away, dsinϑ=mλ will be sufficient to determine maxima for a double slit setup - I believe that the single slit analogue to this equation, asinϑ=mλ, where a is the slit width, gives orders m which are minima.
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