The number of photons emitted in 10 hours by 60 W Sodium lamp of wave length $5893A^0$  is  $x\times {10}^{23}$. What is the value of x (nearest integer)  $h=6.626\times {10}^{-34}Js$. Velocity of light  $=3\times {10}^{8}m{s}^{-1}$

60 watt $\Rightarrow$60 joule in 1 second
60 watt bulb in 10 hours   $60\times 60\times 60\times 60=2160000Joules$
Energy of photon emitted =  $\frac{hc}{\lambda }=\frac{6.626\times {10}^{-34}\times 3\times {10}^8}{5893\times {10}^{-10}}=3.37\times {10}^{-19}J$
No. of photons emitted =  $\frac{216\times {10}^{4}J}{3.373\times {10}^{-19}J}\approx 64\times {10}^{23}$
 $x=64$