The anode voltage of a photocell is kept fixed. The wavelength $\lambda$ of the light falling on the cathode is gradually changed. The plate current $I$ of the photocell varies as follows:

For photoelectric emission to occur, the incident wavelength must be less than the threshold wavelength ($\lambda \le {\lambda }_0$).As $\lambda$ increases, the energy of the photons ($hc/\lambda$) decreases.Once $\lambda >{\lambda }_0$, no photoelectrons are emitted, and the current drops to zero.