Final answer:
A laser requires a power source, an active medium, and mirrors to function, where mirrors reflect photons to enhance stimulated emission of radiation.
Step-by-step explanation:
The three components required to create a laser are a power source, an active medium, and mirrors. The power source, which can be a flash tube or an electrical discharge, provides energy to the active medium in a process known as optical pumping. This leads to a population inversion, which is necessary for laser action. Mirrors are used to enhance the stimulated emission process by reflecting photons multiple times through the active medium, with one mirror being partially transparent to allow the laser beam to exit. The active medium can be made of various materials, including gases, liquids, solids, or semiconductors, and produces the photons necessary for the laser beam.