Question

How many photons are emitted by a laser source of $5 \times 10^{-3} W$ operating at $632.2\, nm$ in $2\, s$ ?

• A. $ 3.2\times 10^{16}$
• B. $ 1.6\times 10^{16}$
• C. $ 4\times 10^{16}$
• D. None of these

Answer 1

Answer: (A)

Energy of each photon
$E_{0}=\frac{h c}{\lambda}$
$=\frac{6.63 \times 10^{-34} \times 3 \times 10^{8}}{632.2 \times 10^{-9}}$
$=3.15 \times 10^{-19} J$
Total energy $=E=P t$
$\therefore n E_{0}=P t$
$\Rightarrow n=\frac{P t}{E_{o}}$
$=\frac{5 \times 10^{-3} \times 2}{3.15 \times 10^{-19}}$
$=3.2 \times 10^{16}$