The number of photons per second on an average emitted by the source of monochromatic light of wavelength 600 nm, when it delivers the power of $3.3 \times 10^{-3}$ watt will be: ($h=6.6\times 10^{-34}$Js)
(1) $10^{18}$
(2) $10^{17}$
(3) $10^{16}$
(4) $10^{15}$
Solution
Let there be n number of photons per second.
Total energy per second of n number of photons = $n. \frac {hc}{\lambda} = 3.3 \times 10^{-3}$
$\therefore n=\frac {3.3 \times 10^{-3} \times 600 \times 10^{-9}}{6.6 \times 10^{-34} \times 3 \times 10^8} =10^{16}$
Answer: (3)