The principle of LASER action involves

The correct answer is option 4) i.e. All of these.

CONCEPT:

LASER - Light Amplification by Stimulated Emission of Radiation.

• A laser is an unusual light source as it is quite different from a light bulb or a flashlight.
• A laser device makes use of atoms or molecules to emit light at particular wavelengths and amplifies that light to produce a narrow beam of radiation.​

​Working principle of laser:

• When the atoms are made incident with light energy, electrons in the lower energy state absorb the energy and go to the excited state. ​After a few milliseconds, these electrons fall back to the ground state by giving off a photon of light.
• Often, these photons excite an already excited atom. When this happens, the excited atom gives off a photon, and the original photon returns. This is called stimulated emission and causes amplification.
• When conditions are created such that there are more atoms having electrons at a higher energy level, more photons will be stimulated than absorbed. This increases the number of photons of the beam. This condition is known as a population inversion.

EXPLANATION:

• A laser beam is produced after stimulated emission, amplification of particular frequency, and population inversion.
• Hence, the principle of LASER action involves all of these.