Lasers

 HOW DO LASERS WORK?

Einstein's work on the photoelectric effect led to the laser, an acronym for "light amplification by simulated emission of radiation." Typically, as electrons are excited from one quantum state to another, they emit a single photon when jumping back. But Einstein predicted that when an already excited atom was hit with the right type of stimulus, it would give off two identical photons. Subsequent experiments showed that certain source materials, such as ruby, not only did that but also emitted photons that were perfectly coherent—not scattered like the emissions of a flashlight, but all of the same wavelength and amplitude.

These powerfully focused beams are now commonplace, found in grocery stores scanners, handheld pointers, and cutting instruments from the hospital operating room to the shop floors of heavy industry.