Calculate the number of photons emitted in 10 hours by a 60 W sodium lamp ($\lambda$ of photon =5893 $\text{Å}$).

Energy emitted by sodium lamp in 1 sec.

=Watt$\times$sec=(60$\times$1)J

Energy of photon emitted $=\frac{hc}{\lambda }$

                                          $=\frac{6.625\times {10}^{-34}x3\times {10}^8}{5893\times {10}^{-10}}=3.37\times {10}^{-19}\mathrm{J}$

power = n .h$\vartheta$

$\therefore$Number of photons emitted per second$=\frac{60}{3.37\times {10}^{-19}}$

$\therefore$Number of photons emitted in 10 hours

         $=17.8\times {10}^{19}\times 10\times 60\times 60=6.40\times {10}^{24}$