Practice question

**Photon power** light source emits 1.0×10¹⁶ photons/s power 4.0 mW, energy per photon E= P/N =4×10⁻³/10¹⁶=4×10⁻¹⁹ J, wavelength λ= h c/E=6.63×10⁻³⁴×3×10⁸/4×10⁻¹⁹=497 nm, for 6×10¹⁵ photons/s 3.0 mW E=5×10⁻¹⁹ J, for beam 2.5×10¹⁵ photons/s 4×10⁻¹⁹ J each power=10⁻³ W=1 mW, illustrating P = N h f. Work function Φ₀ = 2.0 eV = 2.0 × 1.6 × 10⁻¹⁹ = 3.2 × 10⁻¹⁹ J . Threshold frequency v₀ = (Φ₀/h) = (3.2 × 10⁻¹⁹/6.63 × 10⁻³⁴) ≈ 4.83 × 10¹⁴ Hz . Applying E = h f = h c/λ, p = h/λ, K_max = h f - Φ, Φ = h f₀ = h c/λ₀, λ