Question

Number of photons having wavelength $632.8nm$, emitted by $5mW$ laser source in $1$ second is

• A. $1.6\times 10^{19}$
• B. $1.6\times 10^{16}$
• C. $1.6\times 10^{25}$
• D. $1.6\times 10^{13}$

Answer 1

Answer: (B)

$E=\frac{nhc}{\lambda }$
$5\times 10^{-3}J/s=n\times \frac{1240}{632.8}\times 1.6\times 10^{-19}J$
$n=1.6\times 10^{16}$