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)

$\lambda=667 \times 10^{-9} m , P =9 \times 10^{-3} W$
$P =\frac{ Nhc }{\lambda}, N =$ No. of photons emitted /sec.
$N =\frac{9 \times 10^{-3} \times 667 \times 10^{-9}}{6.6 \times 10^{-34} \times 3 \times 10^{8}}$
$=3 \times 10^{16} / sec$