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Homework Help: Number of lines/cm for the grating

  1. Jul 15, 2007 #1
    The problem is:

    Three, and only three, bright fringes can be seen on either side of the central maximum when a grating is illuminated with light ( = 520 nm). What is the maximum number of lines/cm for the grating?

    Here is what i know / did:

    since there are three fringes, m = 3

    since they are bright fringes, it's constructive interference so (m lambda)

    we know that d sin(theta) = (m lambda)

    so, d sin(theta) = (3 * 5.20 E-7 meters)

    we know that the highest displacement has to occur when theta is 90 degrees. So, sin of 90 gives you 1.

    d sin(90) = (3 * 5.2 E -7); D = 1.56 E-6 meters <- This is the slit separation, I am stuck at this point.
    Last edited: Jul 15, 2007
  2. jcsd
  3. Jul 16, 2007 #2
    Never Mind

    I figured it out

    Nmax = 1 / D min

    since d = 1.54 E-6 m = 1.5 E -4 cm

    1 / 1.54 E -4 cm = 6410 lines/cm.
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