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:

In the photoelectric effect, electrons are only emitted if the energy of the incident photon ($E=\frac{hc}{\lambda }$) is greater than or equal to the work function ($\varphi$).This means there is a maximum threshold wavelength ${\lambda }_0=\frac{hc}{\varphi }$.For $\lambda >{\lambda }_0$, no electrons are emitted, and current $I=0$.For $\lambda \le {\lambda }_0$, electrons are emitted. If intensity is constant, the number of photons per second changes with $\lambda$. However, the standard behavior in this setup is that current is maintained until the threshold is crossed, then drops to zero.