(a)[2]
Using the photoelectric effect, state the meaning of the threshold frequency.
(b)[4]
The surface of a zinc plate has a work function of $5.8 \times 10^{-19}\,\text{J}$. In one laboratory experiment, ultraviolet light with wavelength $120\,\text{nm}$ is shone onto the zinc plate. A photoelectric current $I$ is observed. To make the apparatus easier to see, a second lamp that emits light of wavelength $450\,\text{nm}$ is switched on. The ultraviolet lamp is left unchanged. Using suitable calculations, state and explain what happens to the photoelectric current when this second lamp is switched on.