Question

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

• A. $3\times {10}^{16}$
• B. $3\times {10}^{19}$
• C. $9\times {10}^{15}$
• D. $9\times {10}^{17}$

Answer 1

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

Wavelength of light $\lambda =667nm$
Energy of incident photon $E=\frac{hc}{\lambda }$
$⟹E=\frac{6.626\times {10}^{-34}\times 3\times {10}^8}{667\times {10}^{-9}}=3\times {10}^{-19}J$
Power of light $P=9mW=0.009W$
So, number of photons per second $N\frac{P}{E}=\frac{0.009}{3\times {10}^{-19}}=3\times {10}^{16}$ photons/ second