Hand out, for the students to keep, 1"x2" pieces of replica diffraction grating material. Look at the bulb with the diffraction grating about one inch in front of your eye as the bulb warms up to see the following: (1) Several relatively weak lines are initially seen, both from sodium and from mercury, which is used as a seed to get the lamp started (top spectrum). (2) As the bulb warms up collision broadening of the lines occurs, so the weak lines become much brighter and spread out to form a nearly continuous spectrum (second and third spectra). (3) When the lamp is operating at full temperature, "cooler" sodium vapor around the periphery of the bulb absorbs light at the frequency of the sodium doublet, producing a nice dark absorption line (bottom spectrum). In the bottom spectrum the exposure has been reduced using a neutral density filter so that the bright areas and the absorption line are not washed out by overexposure.
The photo above shows the second-order spectrum, so some of the red from the adjacent first-order spectrum causes the blue and violet region of the spectrum shown to be slightly magenta (R+B) colored in some of the individual photographs where the intensity has been adjusted.