Question

How many photons are emitted per second by a $5mW$ laser source operating at $632.8nm$ ?

• A. $1.6\times 10^{16}$
• B. $1.6\times 10^{13}$
• C. $1.6\times 10^{10}$
• D. $1.6\times 10^3$

Answer 1

Answer: (A)

$E=\frac{hc}{\lambda }=\frac{6.6\times 10^{-34}\times 3\times 10^8}{632.8\times 10^{-9}}J$
Number of photons per second, $n=\frac{P}{E}$
$\Rightarrow n=\frac{5\times 10^{-3}}{3.14\times 10^{-19}}$
$=1.6\times 10^{16}$