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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.

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# Homework Help: Number of lines/cm for the grating

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