Diffraction gratings
Diffraction gratings are glass plates with
many parallel lines etched into the surface. Any two adjacent lines can be considered as a
two slit setup, and the forumla, 
applies to find
the angles where a maximum occurs in the interference pattern. The difference is that a
much sharper image results. Consider an angle, just off the maximum, such that the path
difference between the two paths is just one degree off the 360 degrees needed for a
maximum. If one then considers the contribution from a slit 180 etchings away, that
contribution will exactly cancel the first slit, and the intensity falls all the way to
zero. Since the resulting lines are so sharp, a diffraction grating can be used to
precisely measure wavelengths.