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I was reading Lasers and i read something like this:

for spontaneous emission, the rate of decay of electrons in high-energy state to low energy state is given by:

[itex]\frac{dN_2}{dt} = P_{21}N_2[/itex]

where P

_{21}is equal to the probability of electron to drop from higher energy state to lower energy state.

Now, P

_{21}was defined as [itex]P_{21} = B_{21}u(\nu)[/itex]

And [itex]B_{21}[/itex] is called Einstein's Coefficients of spontaneous emission

then in some other topic, they wrote B

_{21}as probability of spontaneous emission per unit time.

Are they really same thing? ... I guess not ... B

_{21}should be something like probability of spontaneous emission per-unit Energy density (:p i know this doesn't make any sense ... ) ...

Please Help ...