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:

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(1) 3×10¹⁶. Energy of one photon: E=(hc)/λ, with hc=12400 eV ångström and λ=6670 ångström. E=(12400)/(6670)=1.86 eV=1.86×1.6×10⁻¹⁹=2.98×10⁻¹⁹ J. The beam delivers P=9×10⁻³ J each second. N=P/E=(9×10⁻³)/(2.98×10⁻¹⁹)=3×10¹⁶ photons per second.

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:

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