A Textbook of Engineering Physics by Avadhanulu, Kshirsagar says the following about Spontaneous Emission:
Photons of slightly different frequencies are generated. As a result, the light is not monochromatic.
It says the following about Stimulated Emission:
The spread of photon frequencies is relatively very narrow. As such the light is nearly monochromatic.
I do not understand why the frequency range of spontaneous emission will be wider and frequency range of stimulated emission will be narrow.
My friend told that the electrons from meta-stable states may also participate in Spontaneous Emission, thus making the light not monochromatic. But wouldn't that hold true in case of Stimulated Emission too?