spectral line
When you spread light into a spectrum, a spectral line is a narrow stripe at one particular colour that stands out from its surroundings — either as a bright bar (an emission line) or a dark notch (an absorption line). The name comes from how they looked in early instruments: a thin slit of light, smeared into a rainbow, showed these features as actual vertical lines.
Each line marks a single, specific jump of an electron between two energy levels in one kind of atom or ion. Because that jump always involves the same energy, it always shows up at the same wavelength. The pattern of lines — their exact positions — is therefore a fixed fingerprint of the element: hydrogen's lines fall in one set of places, helium's in another, iron's in a forest of hundreds. Lines never lie about which element they belong to.
But a spectral line carries far more than identity. Its wavelength can be shifted by motion (a Doppler shift), revealing how fast the source moves toward or away from us. Its strength reveals how much of that element is present and how hot the gas is. Its width reveals temperature, pressure, and spin. Reading the position, depth, and shape of each line is the whole craft of decoding starlight — the barcode of the cosmos.
The famous yellow line of sodium at 589 nm appears bright in a flame test but dark in the Sun's spectrum — same line, same atom, opposite appearance.
One line, one electron jump, one element — but it carries motion, abundance, and temperature too.
No spectral line is infinitely thin: every line has a real width and shape, and that width is itself a measurement, not an imperfection — it encodes the gas's temperature, density, and motion.