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# Monochromatic light of wavelength 632.8 nm is produced by a helium-neon laser. The power emitted is 9.42 mW. How many photons per second, on the average, arrive at a target irradiated by this beam ? (Assume the beam to have uniform cross - section which is less than the target area )

$\begin {array} {1 1} (a)\;0.003 \times 10^{19}photon/s & \quad (b)\;30 \times 10^{16}photon/s \\ (c)\;3 \times 10^{15}photon/s & \quad (d)\;0.33 \times 10^{16}photon/s \end {array}$

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A)
The energy of each photon $E = hc/ \lambda$
$=( 6.626 \times 10^{-34} \times 3 \times 10^8 ) / 632.8 \times 10^{-9} = 3.131 \times 10^{-19}J$
Number of photons arriving per second, at a target irradiated by the beam = $n$
Equation of power, $p = nE$
So, $n = P/E$
$=(9.42 \times 10^{-3}) / (3.141 \times 10^{-19} )=3 \times 10^{-16}$ photon/s = $0.003 \times 10^{19}$ photon/s