JEE Main · 2013 · 07 Apr 2013 · Offline · Modern Physics

A photocell passes current only when the incident photons carry enough energy to eject electrons, which means the wavelength must stay shorter than the threshold ${\lambda }_0$. For every $\lambda <{\lambda }_0$ emission occurs and, with the collecting voltage held fixed, the current sits at its steady saturation value; the instant $\lambda$ climbs past ${\lambda }_0$ the photons are too feeble and the current drops to zero. The matching graph is the one that holds a constant current up to ${\lambda }_0$ and then falls sharply to nothing.