A2 Daily A Level Physics question

A clean photoemissive cathode is uniformly illuminated by a monochromatic beam from a tuneable LED. The optical power delivered to the cathode is the same in both cases, and the illuminated area is unchanged. Setting 1 uses frequency f_R. Setting 2 uses frequency f_V = 1.5 f_R. The metal’s threshold frequency is f_T = 1.2 f_R. A circuit can measure the photocurrent (with a small collecting field) and the stopping potential. Which statement must be true when switching from f_R to f_V?

1. A At f_R (< f_T) there is no emission; at f_V (> f_T) emission occurs. For equal power, the photon arrival rate at f_V is 2/3 of that at f_R, so the photocurrent at f_V is proportional to this lower rate; the stopping potential rises from 0 to a value set by 0.3 of the f_R photon energy per electron. (correct)
2. B Emission occurs at both settings because the power is the same; switching to f_V makes the current 1.5 times larger while the stopping potential is unchanged since total power is fixed.
3. C No emission occurs at either setting because the power per photon is too small; increasing frequency at fixed power changes only how many photons arrive, not their energy or the stopping potential.
4. D Emission occurs only at f_R because it has the higher photon arrival rate; at f_V the current falls to zero and the stopping potential is smaller because fewer photons arrive each second.