Q.What are the characteristics of laser ?

Laser (Light Amplification by Stimulated Emission of Radiation) light differs from ordinary light in four key respects — monochromaticity, coherence, directionality, and intensity — all of which follow from the stimulated-emission process that produces it.

Ordinary light sources such as a sodium lamp emit photons spontaneously and randomly, in all directions, with a spread of wavelengths and with no fixed phase relationship between photons. A laser, by contrast, produces light through stimulated emission: an incoming photon of exactly the transition energy $h\nu = E_2 - E_1$ stimulates an excited atom to emit a second photon that is an exact copy of the first — same frequency, same phase, same direction, and same polarization. Repeated many times inside an optical cavity (via population inversion and amplification between two mirrors), this gives laser light its special properties.

Highly monochromatic: because stimulated emission occurs only between two sharply defined energy levels, the emitted photons all have essentially the same frequency (the wavelength spread $\Delta\lambda$ is extremely small compared to ordinary light sources).

Highly coherent: since every photon is a stimulated copy of the original, all photons in the beam remain in phase with one another as they propagate — both temporally (phase relationship maintained along the direction of travel) and spatially (phase relationship maintained across the wavefront). This coherence is what allows lasers to produce sharp, stable interference and diffraction patterns.

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