emission line
/ ee-MISH-un line /
An emission line is a single bright thread of one pure colour given off by an element — one stripe in its barcode. When excited atoms shed energy, they do not glow in every colour; they release it as light at one exact wavelength, and that narrow streak of light is an emission line.
More precisely, an emission line corresponds to electrons in many atoms all making the same downward jump between two energy levels, each releasing a photon of the same wavelength. The line appears at a fixed position; its intensity (how bright) rises with how many atoms are making that jump, which is why it can measure concentration.
It matters because picking the right line is half the craft of an analysis: an analyst chooses a line that is bright enough for sensitivity yet free of overlap from other elements. The honest caveat is that the strongest line is not always the best — it can sit on a high background or clash with an interferent's line, so a slightly weaker, cleaner line often wins.
Iron has thousands of emission lines. For a clean copper measurement in an iron-rich steel, an analyst chooses a copper line well away from the dense thicket of iron lines, even if a stronger copper line lies buried among them.
One stripe of the barcode; brightness gives concentration, position gives identity.
An emission line and an absorption line for the same transition sit at the same wavelength — emission glows there, absorption darkens there. A whole set of an element's lines together makes up its atomic spectrum.