Physics -Interference & Diffraction

Given Information: Light of wavelength 631 nm passes through a diffraction grating having 485 lines/mm.

Part A: What is the total number of bright spots (indicating complete constructive interference) that will occur on a large distant screen?
*Answer: 7 (this is correct)

Part B: What is the angle of the bright spot farthest from the center?
*Answer: ???
-I am not sure what equation to use for this part of the problem....

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  1. There will be a bright spot wherever
    d sin A = N *lambda,
    where lambda is the wavelength, d is the line spacing, and N is an integer (including zero).

    (1/485)*10^-3 m sin A = N*631*10^-9 m
    sin A = N* 0.306

    Allowed values of N are +/-1,2,3 and 0
    So there are seven bright spots

    B. The center of the pattern is n=0. Compute the angle for which N = + or - 3 and you will have the answer.

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    posted by drwls
  2. Ooh wonderful! There is another problem similar to it...I am having trouble finding the # of fringes, but I think once I get help with that I can solve for the angle!!!

    GIVEN: Light of wavelength 585 nm falls on a slit 6.66×10−2 mm wide.

    Part A: On a very large distant screen, how many totally dark fringes (indicating complete cancellation) will there be, including both sides of the central bright spot?
    *Answer: ?????

    Part B: At what angle will the dark fringe that is most distant from the central bright fringe occur?
    *Answer: I'm asuming that this will be the equation to use: d*sinA=(N+.5)*L

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