A2 Daily A Level Physics question
A clean metal photocathode is illuminated with monochromatic light of frequency f, producing a stopping potential of 0.60 V. The metal’s threshold frequency is 0.5 f. The light frequency is then doubled to 2f while the intensity is kept the same. What is the new stopping potential? (Assume contact potentials are negligible.)
Answer
The correct answer is D.
Correct: D — 1.80 V. The stopping potential scales with the maximum kinetic energy, which increases with (f − f_threshold); since f_threshold = 0.5f, changing from f to 2f multiplies (f − f_threshold) by (1.5f)/(0.5f) = 3, so 0.60 V × 3 = 1.80 V. A 0.30 V implies an inverse relationship with frequency, which is incorrect above threshold. B 0.60 V assumes stopping potential is unaffected by frequency (confusing it with intensity dependence), which is wrong. C 1.20 V assumes simple proportionality to frequency and ignores the offset from the threshold frequency, so it underestimates the increase.