Question

Monochromatic light of wavelength 667 nm is produced by a helium neon laser. The power emitted is 9mW. The number of photons arriving per sec. 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. $3\times {10}^{15}$

Answer 1

Answer: (A)

$3\times {10}^{19}$

Given; $\lambda =667nm$

$=6670A^o$

$\therefore E_{ph}=\frac{12400ev-A^o}{\lambda \left(A^o\right)}$

$\Rightarrow E_{ph}=\frac{12400}{6670}\times {10}^{-19}\times 1.6Joules$----$\left(1\right)$

and Energy emitted per second = Power

$=9\times {10}^{-3}W$

Hence Number of photons Incident $=\frac{Power}{Energy}$

$=\frac{9\times {10}^{-3}\times 6670}{12400\times 1.6\times {10}^{-19}}$

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