The population inversion in a two-level laser material cannot be achieved...

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The population inversion in a two-level laser material cannot be achieved by optical pumping because

A. the rate of upward transitions is equal to the rate of downward transitions

B. the upward transitions are forbidden but downward transitions are allowed

C. the upward transitions are allowed but downward transitions are forbidden

D. the spontaneous decay rate of the higher level is very low

Answer: Option A


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