Q.LASER action needs (A) High temperature
(B) Semiconductor
(C) High pressure
(D) Number inversion

Laser and Stimulated Emission

LASER stands for Light Amplification by Stimulated Emission of Radiation. Ordinary light from a bulb is emitted by atoms decaying independently and randomly (spontaneous emission), so it is a jumble of wavelengths, phases and directions. A laser is different because its light is produced by stimulated emission.

Stimulated emission

When an atom is already in an excited state, an incoming photon of exactly the right energy can trigger it to drop and emit a second photon that is identical to the first — same frequency, same phase, same direction. One photon becomes two coherent photons; those trigger more, and the light is amplified into an avalanche of clones. This idea traces back to Einstein's 1917 analysis of radiation.

The condition: population inversion

Normally most atoms sit in the ground state, so an incoming photon is far more likely to be absorbed than to stimulate emission. To make emission dominate, more atoms must be in the excited state than the lower one — a non-equilibrium condition called population inversion, achieved by pumping energy in (optical, electrical, etc.). The light bounces between two mirrors (an optical cavity) so it passes through the medium many times, building up before escaping through a partly transparent mirror.

Characteristics …