Q.Define stopping potential.

The stopping potential (also called the cut-off potential), denoted $V_0$, is the specific negative (retarding) potential applied to the collecting electrode in a photoelectric-effect apparatus that is just sufficient to stop even the most energetic photoelectrons emitted from the cathode and reduce the photocurrent to exactly zero. As the retarding potential on the collecting electrode is made increasingly negative starting from zero, the photocurrent does not drop to zero immediately (since photoelectrons are emitted with a whole range of different kinetic energies, and the most energetic ones can still overcome a modest retarding field), but falls progressively as more and more electrons are turned back, reaching zero only once the retarding potential reaches the specific value $V_0$ at which even the fastest photoelectrons are brought to rest exactly at the collecting electrode. …