How many photons does a 3.2 mw helium-neon laser (λ = 633 nm) emit in 1 second? how many photons does a 3.2 helium-neon laser ( = 633 ) emit in 1 second? 7.6×1018 2.5×1018 5.1×1016 1.0×1016?

Answer is: 1.0×10¹⁶.
P (power of laser) = 3,2 mW = 0,0032 W.
λ (wavelength) = 633 nm = 633 · 10⁻⁹m = 6,33·10⁻⁷m.
P = n·h·c÷λ ⇒ n = n=P·λ/h·c.
c - speed of light 3⋅10⁸ m/s.
h - Planck's constant - 6.63⋅10⁻³⁴J·s.
n - number of photons emited.
n = 0,0032W · 6,33·10⁻⁷m / 6.63⋅10⁻³⁴J·s · 3⋅10⁸ m/s
n = 1·10¹⁶ 1/s.