Atomic spectra and the hydrogen spectrum

A prism spreads white light into a continuous spectrum: red bends least, violet most, and each colour merges into the next, as in a rainbow.

An excited atom returns to lower energy by emitting radiation. The emission spectrum records the emitted wavelengths; the absorption spectrum is its photographic negative, with dark lines where the sample took in light.

Gaseous atoms give line spectra, bright lines at specific wavelengths with dark gaps. Each element's pattern is unique, like a fingerprint, and the study of these spectra is spectroscopy.

Spectroscopy found rubidium, caesium, thallium, indium, gallium and scandium, and helium was discovered in the sun this way; Robert Bunsen was an early user of line spectra to identify elements.

An electric discharge splits H₂ and excites the H atoms, which emit a line spectrum. Balmer (1885) fitted the visible lines with ν̄ = 109,677 (1/2² − 1/n²) cm⁻¹ for n = 3, 4, 5 …

Rydberg's general form: ν̄ = 109,677 (1/n₁² − 1/n₂²) cm⁻¹ with n₂ > n₁; 109,677 cm⁻¹ is the Rydberg constant for hydrogen.

Series by n₁: Lyman 1 (ultraviolet), Balmer 2 (visible, the only visible series), Paschen 3, Brackett 4 and Pfund 5 (all infrared).

Hydrogen has the simplest line spectrum; every element's spectrum is unique and shows regularity, which points to its electronic structure.