What does LASER stand for?

The story behind the answer

LASER stands for Light Amplification by Stimulated Emission of Radiation. “Stimulated emission” is the key physics: an incoming photon triggers an excited atom to emit a second photon with the same energy, phase, and direction. In a laser’s optical cavity (mirrors at both ends), these photons bounce and multiply, producing intense, coherent, narrowly focused light—hence “amplification.”

Einstein predicted stimulated emission in 1917; the first working laser was built by Theodore Maiman in 1960 (ruby laser). Relatedly, a MASER uses microwaves instead of visible light. Memory tip: think “Light is Amplified by Stimulating Emission of Radiation” (LASER), where “stimulated emission” is the special process and “radiation” is just a physics term for light.