A photoelectric I–V curve is shown. Work function of metal is 2 eV. With photoemission efficiency 10⁻³%, estimate incident light power (W), using graph data. [See Image.]
Physics · Mixed Chapter
JEE Main Practice

Question

A photoelectric I–V curve is shown. Work function of metal is 2 eV. With photoemission efficiency 10⁻³%, estimate incident light power (W), using graph data. [See Image.]

G1 Physics · Accepted Answer

Step 1 — From I–V graph, stopping voltage is 5 V and saturation photocurrent is 10 μA. Step 2 — Einstein photoelectric equation gives photon energy hν = φ + eVₛ = 2 + 5 = 7 eV. Step 3 — Emitted electrons per second = Iₛ/e = 10 × 10⁻⁶/(1.6 × 10⁻¹⁹) = 6.25 × 10¹³ s⁻¹. Step 4 — Emission efficiency is 10⁻³% = 10⁻⁵. Photon rate incident = 6.25 × 10¹⁸ s⁻¹. Step 5 — Optical power = rate × photon energy = (6.25 × 10¹⁸) × (7 × 1.6 × 10⁻¹⁹) = 7 W.