Atoms can absorb extra energy from heat, an electric spark, or intense light. This added energy excites the electrons and forces them outward into higher energy orbitals. These electron orbitals are strictly quantized.
This means an electron can only exist at specific allowed energy levels. An electron can never hover in the empty space between these distinct levels. Eventually, the unstable electron drops back down to a lower, more stable orbital.
As it falls, it releases its extra energy as a tiny packet of light called a photon. The physical energy of this photon exactly matches the fixed energy gap between the two orbital levels. Every chemical element has a completely unique arrangement of electron energy levels.
Therefore, every element produces a distinct set of photon energies. This creates a specific visual pattern of bright colored lines. Scientists use this unique chemical fingerprint to quickly identify unknown elements in a lab or distant stars.
A hot gas at low pressure emits a line spectrum. This spectrum displays discrete bright lines separated by wide dark gaps. A hot glowing solid will emit a continuous spectrum that contains a smooth rainbow of all visible wavelengths.