Is the laser light truly monochromatic? If not what is the typical line width of a laser?
No, laser light is not truly monochromatic. This line width is decided by various broadening factors such as Doppler effect of moving atoms and molecules. Typically, the frequency bandwidth of a commercial He-Ne laser is about 1500MHz (full width at half-maximum, FWHM). In terms of wavelength, it means that at a wavelength of 632.8nm this means a wavelength bandwidth of about 0.01nm. On the other hand, the bandwidth of a typically diode laser with a wavelength of 900nm is about 1nm as compared to LED, which has a bandwidth of approximately 30 – 60 nm.
