Ordinary light spreads out, scatters, and mixes every color and phase together. A laser is disciplined: one wavelength, one direction, every wave crest marching in step. That discipline comes from a chain reaction called stimulated emission — and it's why a laser can bounce off the Moon and come back readable.
Left alone, an excited atom eventually drops to a lower level at a random moment, emitting a photon in a random direction — spontaneous emission. But if a photon of exactly the right energy passes by first, it can trigger the atom to emit an identical photon immediately — same energy, same direction, same phase. That triggered copy is stimulated emission, and it's the seed of every laser.
Because the emitted photon in stimulated emission is a perfect copy, laser light ends up with four distinctive properties no ordinary lamp can match.
Ordinary light sources emit a broad spread of wavelengths in every direction, out of phase, spreading and weakening with distance. A laser instead emits one sharp wavelength (monochromaticity), as a tight parallel beam (collimation), with every photon in step (coherence) — so its intensity barely drops even over kilometers, instead of falling off with the inverse square law.
Getting light to behave this way requires forcing more atoms into the excited state than the ground state — population inversion.
Normally more atoms sit in the ground state than any excited state. An external energy source (electrical, optical, thermal, or chemical pumping) can invert this, parking more atoms in an excited "metastable" level than in the ground state — population inversion. One stray photon then triggers stimulated emission, and the identical photons it creates trigger yet more emissions, bouncing between two mirrors (the resonant cavity) until the beam is amplified enough to leak out through a semi-transparent mirror as a laser beam.
The helium-neon laser is the classic example of turning this cascade into a steady, practical beam.
A glass tube holds helium and neon gas in a 10:1 ratio at low pressure. An electric discharge excites helium atoms to a level that happens to closely match one of neon's excited levels. Colliding helium atoms transfer their energy to neon, building a population inversion in neon's long-lived metastable state — and neon supplies the 632.8 nm red laser light.
Because the beam is intense, coherent, and razor-focused, it has found uses far beyond the physics lab.
Holography records both the intensity and the phase of light reflected from an object, using a reference beam to capture full 3D information — something only a coherent laser source can do. Beyond holography, lasers are used in medicine (retinal detachment repair, vision correction, surgery with fiber-optic endoscopes), communications (fiber-optic data links), industry (precision cutting and welding), the military (missile guidance, LADAR), CD/DVD recording, laser printing, surveying, and space research — including bouncing a beam off a reflector on the Moon to measure the Earth-Moon distance to within centimeters.