Question

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 average, arrive at a target irradiated by this beam? (Assume the beam to have a uniform cross-section which is less than the target area).

Answer 1

Given $h=6.663\times {10}^{-34}js,\lambda =632.8nm,c=3\times {10}^8,P=9.42mW$

We know the formula,

$E=\frac{hc}{\lambda }=\frac{6.663\times {10}^{-34}\times 3\times {10}^8}{632.8\times {10}^{-9}}$

$E=3.14\times {10}^{-19}J$

Number of photons falling on the target per second,
$n=\frac{P}{E}=\frac{9.42\times {10}^{-3}}{3.14\times {10}^{-19}}=3\times {10}^{-16}{s}^{-1}$