atomic spectrum
When you pass sunlight through a prism you get a smooth rainbow, every colour blending into the next. But heat a particular gas — say neon in a sign — and pass its glow through a prism, and instead of a continuous rainbow you see only a few sharp, isolated bright lines, like a barcode of colours. That barcode is the atomic spectrum, and each element has its own unique one.
An atomic spectrum is the set of specific wavelengths (colours) of light that an element's atoms emit or absorb. Because an electron in an atom can only hold certain fixed energies, it can only jump between those allowed levels — and each jump releases or absorbs a photon of one exact energy, hence one exact colour. The result is a pattern of discrete lines rather than a continuous band.
This is why atomic spectra are so powerful: the line pattern is a fingerprint that identifies an element from a glow alone, even in a distant star. The caveat is that real spectra are richer than a few clean lines — lines can be broadened, split by magnetic fields, or shifted by motion — but the core idea, that quantised energy levels produce discrete lines, holds firmly.
Hydrogen's visible spectrum shows just four bright lines — red, blue-green, blue, and violet — known as the Balmer series. Each one comes from an electron dropping from a higher level down to the second level, and their exact spacing was the clue that first revealed atoms have quantised energy levels.
A barcode of light — each element's spectrum is its unique fingerprint.
Emission spectra show bright lines on a dark background (light given off); absorption spectra show dark lines on a bright background (light taken out). They appear at the same wavelengths for the same element.