Number of photons of equal energy emitted per second by a 6 mW laser source operating at 663 nm i $s _____.$
(Given : h = 6.63 × 10⁻³⁴ J·s and c = 3 × 10⁸ m/s)

Question

Number of photons of equal energy emitted per second by a 6 mW laser source operating at 663 nm i s _____. (Given : h = 6.63 × 10⁻³⁴ J·s and c = 3 × 10⁸ m/s)

G1 Physics · Accepted Answer

Energy of one photon is E(γ) = hc/λ. If n photons are emitted per second, laser power is P = nE(γ) = nhc/λ. Therefore, n = Pλ/(hc). Given, P = 6 mW = 6×10⁻³ W, λ = 663 nm = 663×10⁻⁹ m. Thus, n = [6×10⁻³×663×10⁻⁹]/ [6.63×10⁻³⁴×3×10⁸] = 2×10¹⁶ photons/s. Correct option: D.