A 25 Watt bulb emits monochromatic yellow light of wavelength of 0.57  $\mu$ m. What will be the rate of emission of quanta per second?                                                                                        

$E_{Photon}=\frac{hc}{\lambda }=\frac{6.626\times {10}^{-34}\times 3.0\times {10}^8}{0.57\times {10}^{-6}}$

                             $=34.86\times {10}^{-20}J$

Watt = power = number of photons emitted /s $\times E_{Photon}$

25 = number of photons emitted/ s $\times 34.86\times {10}^{-20}$

 $\therefore$ Number of photons emitted /s   =$\frac{25}{34.86\times {10}^{-20}}$

                                                            $=7.17\times {10}^{19}/s$