What leads to monochromaticity in a laser light?
Laser light consists of essentially one wavelength, having its origin in stimulated emission from one set of atomic energy levels. This is possible because laser transition, in principle, involves well-defined energy levels. EM wave of frequency n = (E2 – E1) only can be amplified, n has a certain range which is called line width. This line width is decided by various broadening factors such as Doppler effect of moving atoms and molecules. The generation of laser is such that the laser cavity forms a resonant system and laser oscillation is sustained only at the resonant frequencies of the cavity. This leads to the further narrowing of the laser line width. So laser light is usually very pure in wavelength, we say it has the property of monochromatic.
