AS Daily A Level Physics question
A student measures the stopping potential for a clean metal photocathode using a vacuum photocell. With 405 nm violet light, the stopping potential is 1.0 V. With 650 nm red light, no photoelectrons are detected at any intensity. Using photon energy (eV) ≈ 1240/λ(nm), which value is most consistent with the metal’s work function?
Answer
The correct answer is D.
Correct: D — 2.1 eV. From 405 nm: photon energy ≈ 1240/405 ≈ 3.06 eV, so work function ≈ 3.06 − 1.0 ≈ 2.06 eV ≈ 2.1 eV; 650 nm gives ≈ 1.91 eV, which is below this value, explaining no emission. A 1.0 eV — confuses stopping potential energy (eVs) with the work function; it would not explain the 405 nm result. B 1.9 eV — equals the 650 nm photon energy; if this were the work function, 650 nm would just emit, contradicting the observation. C 3.1 eV — equals the 405 nm photon energy; this would predict zero stopping potential, contradicting the 1.0 V measurement. D 2.1 eV — consistent with both the 405 nm stopping potential and the absence of emission at 650 nm.