If the atom in the excited state automatically decays to the ground state by emitting a photon of energy (E2 – E1 = hυ), then this process is called spontaneous emission. Generally an atom/electron in excited state can stay for
10-9 – 10-8 seconds.
Characteristics of spontaneous emission:
a) The emitted photon of energy E2 – E1 = hυ can move in any random direction.
b) There will not be any phase relationship between the photons emitted from various atoms.
Hence, the radiations coming out due to spontaneous emissions are incoherent.
Reference: The content of all the articles of lasers posted here is referred from my book: Optics and Lasers by Amit Sarin and Sameer Kalia, published by kalyani publishers.
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