Q. 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) $9 \times 10^{17}$
B) $3 \times 10^{16}$
C) $9 \times 10^{15}$
D) $3 \times 10^{19}$
Explanation
(b) $\frac{n}{t} = \frac{P\lambda}{hc} = \frac{(9 \times 10^{- 3}) \times (667 \times 10^{- 9})}{6.6 \times 10^{- 14} \times 3 \times 10^{8}} = 3 \times 10^{16}$
Reviewed by Dr. Abubacker
Physics Educator & NEET/JEE Specialist
Ph.D. in Physics with 19+ years of teaching experience.