Previous Year Question

LASER stands for Light Amplification by Stimulated Emission of Radiation. Thus, the fundamental principle of a laser is Stimulated Emission of Radiation.

In stimulated emission, an atom or molecule is initially in an excited state. When a photon of appropriate energy interacts with the excited atom, it stimulates the atom to move to a lower energy state. During this transition, another photon is emitted.

An important characteristic of this process is that the newly emitted photon has the same frequency, phase and direction as the stimulating photon. Repeated stimulated emission produces amplification of light and generates a highly coherent, directional and nearly monochromatic beam.

In spontaneous radiation, an excited atom emits radiation without being triggered by an external photon, so it is not the principal mechanism of laser action. Similarly, dispersion separates light into different wavelengths, whereas scattering changes the direction of light.