<Dl> <Dd> <Table> <Tr> <Td> (d n 1 d t) s p o n = A 21 n 2 (\ displaystyle \ left ((\ frac (dn_ (1)) (dt)) \ right) _ (\ mathrm (spon)) = A_ (21) n_ (2)) </Td> <Td> Spontaneous emission </Td> </Tr> <Tr> <Td> (d n 1 d t) s t i m = B 21 n 2 u ν (\ displaystyle \ left ((\ frac (dn_ (1)) (dt)) \ right) _ (\ mathrm (stim)) = B_ (21) n_ (2) u_ (\ nu)) </Td> <Td> Stimulated emission </Td> </Tr> <Tr> <Td> (d n 2 d t) a b s = B 12 n 1 u ν (\ displaystyle \ left ((\ frac (dn_ (2)) (dt)) \ right) _ (\ mathrm (abs)) = B_ (12) n_ (1) u_ (\ nu)) </Td> <Td> Photo - absorption </Td> </Tr> </Table> </Dd> </Dl> <Dd> <Table> <Tr> <Td> (d n 1 d t) s p o n = A 21 n 2 (\ displaystyle \ left ((\ frac (dn_ (1)) (dt)) \ right) _ (\ mathrm (spon)) = A_ (21) n_ (2)) </Td> <Td> Spontaneous emission </Td> </Tr> <Tr> <Td> (d n 1 d t) s t i m = B 21 n 2 u ν (\ displaystyle \ left ((\ frac (dn_ (1)) (dt)) \ right) _ (\ mathrm (stim)) = B_ (21) n_ (2) u_ (\ nu)) </Td> <Td> Stimulated emission </Td> </Tr> <Tr> <Td> (d n 2 d t) a b s = B 12 n 1 u ν (\ displaystyle \ left ((\ frac (dn_ (2)) (dt)) \ right) _ (\ mathrm (abs)) = B_ (12) n_ (1) u_ (\ nu)) </Td> <Td> Photo - absorption </Td> </Tr> </Table> </Dd> <Table> <Tr> <Td> (d n 1 d t) s p o n = A 21 n 2 (\ displaystyle \ left ((\ frac (dn_ (1)) (dt)) \ right) _ (\ mathrm (spon)) = A_ (21) n_ (2)) </Td> <Td> Spontaneous emission </Td> </Tr> <Tr> <Td> (d n 1 d t) s t i m = B 21 n 2 u ν (\ displaystyle \ left ((\ frac (dn_ (1)) (dt)) \ right) _ (\ mathrm (stim)) = B_ (21) n_ (2) u_ (\ nu)) </Td> <Td> Stimulated emission </Td> </Tr> <Tr> <Td> (d n 2 d t) a b s = B 12 n 1 u ν (\ displaystyle \ left ((\ frac (dn_ (2)) (dt)) \ right) _ (\ mathrm (abs)) = B_ (12) n_ (1) u_ (\ nu)) </Td> <Td> Photo - absorption </Td> </Tr> </Table> <Tr> <Td> (d n 1 d t) s p o n = A 21 n 2 (\ displaystyle \ left ((\ frac (dn_ (1)) (dt)) \ right) _ (\ mathrm (spon)) = A_ (21) n_ (2)) </Td> <Td> Spontaneous emission </Td> </Tr>

Planck's quantum theory was based on observations of which of the following