Question

How many photons are emitted per second by $a5mW$ laser source operating at $632.8nm$?

Answer 1

Wavelength $=632.8nm$

$=6328A^o$

Power $=5mW$

$\therefore E=\frac{hc}{\lambda }$

$=\frac{12375}{\lambda \left(A^o\right)}$

$\therefore$ Energy $=\frac{12375}{6328}$

$=1.955eV$

$\therefore$ Energy of photon in eV $=1.955\times 1.6\times {10}^{-19}$

$=3.12895\times {10}^{-19}$eV

Let no. of photons emitted in one second $=n$

$\therefore$ No. of photons emitted per second

$n=\frac{Power}{Energyofphoton}$

$=\frac{5\times {10}^{-3}}{3.1289\times {10}^{-19}}$

$=1.597\times {10}^{16}$