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# Consider two light sources of wavelengths $\lambda_1\: and\: \lambda_2 (\lambda_1 > \lambda_2)$ which are emitting $n_1\: and \: n_2$ photons respectively, in a given time. Assume equal power for both sources, then

$\begin {array} {1 1} (a)\;n_1 > n_2 & \quad (b)\;n_1 < n_2 \\ (c)\;n_1 = n_2 & \quad (d)\;Can’t\: be\: determined \end {array}$

Ans : (a) $n_1 > n_2$
Power = energy/time = $[n_1(hc/\lambda_1)]/ \Delta\: t$
$= [n_2(hc/\lambda_2)]/\Delta\: t$
$\Rightarrow n_1/\lambda_1 = n_2/\lambda_2$
As, $\lambda_1/\lambda_2 > 1$
So, $n_1 > n_2$

edited Mar 13, 2014