Question

Visible light is incident perpendicularly on a diffraction grating of 200 rulings/mm. What are the (a) longest, (b) second longest, and (c) third longest wavelengths that can be associ-ated with an intensity maximum at theta=30 degrees?

Answer 1

Since the longer the wave the farther apart the maxima are, the question boils down to what wavelengths of light has first-, second-, and third-order mode maxima at 30 degrees.

Apply the formula for maxima found in the following link.

http://en.wikipedia.org/wiki/Diffraction_grating#Theory_of_operation