SPONTANEOUS AND STIMULATED EMISSION OF RADIATION.

Spontaneous emission of radiation:

This is the phenomenon of emission of energy in the form of radiation when an atom jumps from excited energy state to the lower energy state after completing its life time in the excited state.

Explanation:

The atoms of a substance are excited to higher energy state from ground state by absorbing an amount of energy indenting on it.

But the atom in the excited level is not stable, because the life time of the atom in the excited state is about 10^-9 sec ( nano second).

After 10^-9 seconds the atoms jump down to the lower energy level releasing an amount of radiation.

The amount of energy released is equal to the difference in the two energy states.

Let the atom is excited to a higher energy state E2 and jumps to lower energy state E1 after its life time.

Then the amount of energy release E = hc/λ= E2-E1

Where λ= wavelength of the radiation and c= velocity of light and h = Planck's constant.
In this transition each atom release a photon of energy hc/λ.
The radiation released in spontaneous emission is not a monochromatic and coherent. The reason is all the atoms do not reach the same lower energy level and hence the radiation consists different wavelengths.

Stimulated emission of radiation:

This the phenomenon of transition of an excited atom to jump to the lower energy state before its life time 10 ^ -9 sec.
when an external photon incidents on it. In general the excited photon has life time equal to 10^-9 sec. in the higher energy state.

After this time the atom spontaneously jumps to lower energy state releasing a photon.
In the process of stimulated emission an external photon (selected wavelength) of energy hc/λ incidents on the excited atom and stimulates the atom to jump from excited state to lower energy state before its life time 10^-19 sec.

During this transition the atom releases two equal photons which are equal to incident photon hc/λ . The two released photons are exactly identical to the incident photon.

for explanation :

https://youtu.be/5cp7UsKw3hw

https://youtu.be/j8pjlS7hN08

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