Final answer:
Photons in a laser induce stimulated emission in atoms, creating additional photons of the same frequency and phase, which results in coherent and monochromatic light.
Step-by-step explanation:
During the operation of a laser, as photons interact with atoms inside the laser, the photons stimulate those atoms to emit additional photons through a process known as stimulated emission. The original and the newly created photons have the same frequency and phase, leading to coherent and monochromatic light. The photons are initially absorbed by the electrons in the atoms, which are then elevated to higher energy levels. Most of these electrons drop back to their ground state immediately, while some remain in a metastable state. A population inversion is created when more atoms are in the excited state than in the ground state. When a photon with the correct energy interacts with an electron in the metastable state, it triggers the emission of a second photon with the same energy, contributing to the coherence of the laser light. The laser light amplification is thus derived from the energy input, such as from a flash tube or an electrical discharge, leading to a cascade or chain reaction of photon production.