When salt is sprinkled on a flame, yellow light consisting of two closely spaced wavelengths, 588.997 nm and 589.594 nm, is produced. They are called the sodium D lines and were observed by Fraunhofer in the Sun's spectrum.
(a) If this light falls on a diffraction grating with 300 lines per millimeter, what is the angle between the second-order spectra of these two wavelengths?
(b) How many lines of this grating must be illuminated for the sodium D lines to just be resolved?