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).

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Solution

Given $h = 6.663 \times 10^{- 34} j s, λ = 632.8 n m, c = 3 \times 10^{8}, P = 9.42 m W$
We know the formula,
$E = \frac{h c}{λ} = \frac{6.663 \times 10^{- 34} \times 3 \times 10^{8}}{632.8 \times 10^{- 9}}$
$E = 3.14 \times 10^{- 19} J$
Number of photons falling on the target per second,
$n = \frac{P}{E} = \frac{9.42 \times 10^{- 3}}{3.14 \times 10^{- 19}} = 3 \times 10^{- 16} s^{- 1}$

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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 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).

Given h=6.663×10−34js,λ=632.8nm,c=3×108,P=9.42mWWe know the formula,E=hcλ=6.663×10−34×3×108632.8×10−9E=3.14×10−19JNumber of photons falling on the target per second,n=PE=9.42×10−33.14×10−19=3×10−16s−1

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).