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)

The 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 $=E=Pt$
$\therefore nE_{0}=Pt$
$\Rightarrow n=\frac{P t}{E_{0}}=\frac{5 \times 10^{- 3} \times 2}{3.15 \times 10^{- 19}}=3.2\times 10^{16}$