A laser of power 1.0 mW emits light of wavelength 633 nm. About how many photons does it emit per second (h = 6.63 × 10⁻³⁴ J s, c = 3.0 × 10⁸ m/s)?
Correct answer B. 3.2 × 10¹⁵
Explanation
Energy per photon = hc/λ = 3.14 × 10⁻¹⁹ J. Number per second = P/E = 1.0 × 10⁻³/3.14 × 10⁻¹⁹ ≈ 3.2 × 10¹⁵.
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