An Example
Light with l = 500 nm is incident on two narrow slits separated by d = 30 mm. An interference pattern is observed on a screen a distance L away from the slits. The first dark fringe is found to be 1.5 cm from the central maximum. Find L.
B.S. Ambrose, P.S. Shaffer, R.N. Steinberg, and L.C. McDermott, Am. J. Phys. (1998)