Newton showed that when white light is passed through a prism it is split into a spectrum of all colors from violet through red. There were no gaps between the colors, so the spectrum is called continuous.

The emission line of sodium has only two yellow lines. Hydrogen has four lines. The spectrum of iron, on the other hand, has an extremely large number of lines.

An absorption-line spectrum occurs when the low-density gas absorbs distinct colors, leaving dark gaps in the otherwise continuous spectrum. For example, the spectrum that the German physicist Joseph von Fraunhofer (1787-1826) took of the spectrum of the sun in 1814 showed 574 dark lines. In 1859 they were explained as being absorption lines from the cooler gases in the sun’s atmosphere.

Fraunhofer made the best optical glass of any glassmaker of his era. He made great improvements to the achromatic lens, which refracts light of all colors the same amount. Like most glassmakers of his era, he died young, most likely from the poisonous effects of the materials used to make the glass.