Every element, when heated or excited, glows with its own signature set of colors — a fingerprint no two elements share. Bohr explained why: electrons can only orbit at certain fixed energies, and light is emitted only when they jump between them. Push those jumps hard enough, and you get X-rays instead of visible light.
Bohr proposed that an electron can only occupy discrete circular orbits, each with a fixed energy En = −13.6/n² eV. The electron radiates no energy while in one of these stationary orbits. It emits or absorbs a photon of energy hf = Ei − Ef only when it jumps between two allowed orbits.
Every jump down to the same final level produces the same photon energy every time — grouping all the possible jumps into named series.
Jumps that end on n=1 form the Lyman series (ultraviolet). Jumps ending on n=2 form the Balmer series (mostly visible light) — the one Balmer first noticed as colored lines in hydrogen's spectrum. Higher final levels (Paschen, Brackett, Pfund) fall further into the infrared.
A spectrometer is the instrument that actually spreads this light into its component wavelengths so we can read them off.
Light from a glowing gas passes through a narrow slit, is collimated into parallel rays, then spread by a prism or diffraction grating according to wavelength. A telescope on a rotating arm reads off the angle of each colored line, letting the wavelengths be measured precisely and compared against known spectral series.
Turn the process around — accelerate electrons into a metal target instead of exciting a gas — and the same energy-jump idea produces X-rays.
In a Coolidge tube, electrons boiled off a hot filament are accelerated by a high voltage and smashed into a metal anode. Most electrons decelerate gradually, radiating a continuous spread of X-ray wavelengths (bremsstrahlung) down to a sharp minimum wavelength set by the tube voltage. A few electrons instead knock out an inner-shell electron of the target atom, and the atom's own characteristic X-ray lines appear as the vacancy is refilled.
Applications: medical imaging and cancer therapy, airport baggage screening, industrial flaw detection in metal castings, and crystallography for determining atomic structure.