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Monochromatic light of wavelength 667 nm is produced by a helium neon laser. The power emitted is 9 mW. The number of photons arriving per second on the average at a target irradiated by this beam is
A
$3\times10^{19}$
B
$9\times10^{17}$
C
$3\times10^{16}$
D
$9\times10^{15}$
Detailed Solution
Energy of one photon: $E = \frac{hc}{\lambda} = \frac{6.63\times10^{-34}\times3\times10^8}{667\times10^{-9}}$
$E = 2.98\times10^{-19}$ J $\approx 3\times10^{-19}$ J
Power = number of photons per second × energy of each photon: $P = nE$
$n = \frac{P}{E} = \frac{9\times10^{-3}}{3\times10^{-19}}$
$n = 3\times10^{16}$ photons per second
$E = 2.98\times10^{-19}$ J $\approx 3\times10^{-19}$ J
Power = number of photons per second × energy of each photon: $P = nE$
$n = \frac{P}{E} = \frac{9\times10^{-3}}{3\times10^{-19}}$
$n = 3\times10^{16}$ photons per second