A2 Daily A Level Physics question
In a lab, a clean metal photocathode with threshold frequency f0 is illuminated by monochromatic light of frequency 1.20 f0 and intensity I. The magnitude of the reverse potential needed to reduce the photocurrent to zero is measured as Vs1. The light frequency is then increased by 10% (to 1.32 f0) while the intensity is halved to I/2. Which statement must be true about the new stopping potential magnitude Vs2?
Answer
The correct answer is A.
Correct: A — Vs2 = 1.6 Vs1; the change in intensity does not alter this. The stopping potential magnitude depends on the excess photon energy above the threshold, so Vs ∝ (f − f0); here (1.32 − 1.00)/(1.20 − 1.00) = 0.32/0.20 = 1.6, and intensity does not change the maximum electron energy. B is wrong because intensity affects the number of emitted electrons (current) not their maximum energy, so it cannot cancel the frequency increase. C is wrong because it assumes proportionality to f rather than (f − f0), underestimating the change near threshold. D is wrong because it falsely claims intensity changes electron energy; intensity only changes the emission rate, so lowering intensity cannot reduce the stopping potential required for the most energetic electrons.