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 second on the average at a target irradiated by this beam is :

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

Answer 1

Answer: (C)

$3\times {10}^{16}$

$No.ofphotons=\frac{E}{\left(hc/\lambda \right)}$
$=\frac{9\times {10}^{-3}\times 6.67\times {10}^{-7}}{6.6\times {10}^{-34}\times 3\times {10}^8}$

$=3\times {10}^{16}$