emission-line spectrum
Look at a neon sign, a sodium street lamp, or a glowing cloud of gas in space, and pass its light through a prism. Instead of a smooth rainbow you see only a few bright, sharp lines of pure colour against a dark background, like a handful of coloured bars on a black ruler. That picture is an emission-line spectrum, and the bright lines are the precise colours that element loves to emit.
It happens whenever a thin, hot gas glows on its own. Heat or electricity kicks the electrons up to higher energy levels; as they tumble back down, each jump throws out a photon of one exact colour. Because the gas is thin, those photons escape straight to your eye without being reabsorbed, so you see bright lines exactly where the atoms emit and darkness everywhere else. Neon's pattern is its own, hydrogen's is different, mercury's different again.
In astronomy these bright-line spectra come from glowing nebulae, the outer atmospheres of the Sun, and the gas swirling around hot young stars. Reading which lines are present tells astronomers which elements are glowing; how bright each line is hints at how much gas there is and how hot and dense it is. An emission-line spectrum is a thin hot gas announcing its chemistry in lights.
A glowing cloud of hydrogen — like the Orion Nebula — shines mainly at hydrogen's red 656 nm line, giving many nebulae their characteristic rosy glow in photographs.
Bright lines on darkness: a thin hot gas emitting only its own colours.
Bright emission lines and dark absorption lines come from the very same electron jumps in the very same element — which one you see depends only on whether the gas is glowing by itself or sitting in front of a brighter background.