A2 Daily A Level Physics question
In a vacuum photocell, a clean metal surface is illuminated with monochromatic light of frequency f1. The measured stopping potential is 1.0 V. The metal’s threshold frequency is 0.8 f1. The light intensity is unchanged. If the light frequency is increased to 1.2 f1, what is the new stopping potential?
Answer
The correct answer is D.
Correct: D — 2.0 V. The stopping potential is proportional to how far the light frequency is above the threshold: at f1 the excess is f1 − 0.8f1 = 0.2f1; at 1.2f1 it is 1.2f1 − 0.8f1 = 0.4f1, which is double, so the stopping potential doubles from 1.0 V to 2.0 V. A 1.0 V — wrongly suggests no change, as if stopping potential depended only on intensity; here intensity is unchanged and the higher frequency increases electron energy. B 1.2 V — assumes direct proportionality to frequency f, ignoring the offset from the threshold frequency. C 1.5 V — arises from an incorrect ratio (1.2/0.8), not from the correct difference-from-threshold reasoning. D 2.0 V — correct because the excess frequency doubles (0.4 vs 0.2), so the stopping potential doubles.