Question

Monochromatic light of wavelength 667 nm is produced by a laser lamp. The power emitted is 9 mW. The number of photons arriving per second on the average at a target irradiated by this beam is

• A. $3\times {10}^{16}{s}^{-1}$
  
• B. $9\times {10}^{16}{s}^{-1}$
  
• C. $3\times {10}^{19}{s}^{-1}$
  
• D. $9\times {10}^{17}{s}^{-1}$
  

Answer 1

The correct option is A $3\times {10}^{16}{s}^{-1}$


Energy of emitted photon,
$E=\frac{hc}{\lambda }=\frac{12400}{6670}=1.86eV$
Number of photons arriving per second on the target is
$=\frac{P}{E}=\frac{9\times {10}^{-3}}{1.86\times 1.6\times {10}^{-19}}=3\times {10}^{16}{s}^{-1}$