(a) A discharge tube containing unknown gas shows bright lines at 656 nm, 486 nm and 434 nm. Identify the gas. (b) The Sun's spectrum shows a dark line at 589 nm. What does this tell us, and which element is responsible? Solution: (a) These are H-alpha (656), H-beta (486), H-gamma (434) — the Balmer series of hydrogen. The gas is hydrogen. (b) A dark line at 589 nm means the Sun's cooler atmosphere absorbs 589 nm photons from the hotter interior's continuous spectrum. 589 nm is the sodium D-line pair — so sodium exists in the Sun's atmosphere. Absorption lines reveal composition without us ever visiting the Sun.