Question

Monochromatic light of wavelength $667\, nm$ is produced by a helium neon laser. The power emitted is $9\, mW.$ The number of photons arriving per sec. on the average at a target irradiated by this beam is

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

Answer 1

Answer: (A)

No. of photons $=\frac{E}{(\text { hc } / \lambda)}$
$=\frac{9 \times 10^{-3} \times 6.67 \times 10^{-7}}{6.6 \times 10^{-34} \times 3 \times 10^{8}}$
$=3 \times 10^{16}$