A2 Daily A Level Physics question

In a vacuum photocell, a clean sodium cathode (work function 2.3 eV) is illuminated with monochromatic 300 nm light, giving a stopping potential about 1.8 V. The lamp is replaced with a 600 nm source of the same intensity. What is the new stopping potential? Support your answer with a short numerical estimate.

1. A 0 V, because the photon energy at 600 nm (~2.1 eV) is below the 2.3 eV work function so no electrons are emitted. (correct)
2. B About 0.9 V, roughly half, since doubling wavelength halves frequency and thus halves the stopping potential.
3. C About 1.8 V, unchanged, because the intensity stayed the same so the electron energy is unchanged.
4. D A small negative value (around −0.9 V), because longer-wavelength light gives slower electrons that need reverse bias to be collected.