In the photoelectric effect, for a given intensity and frequency of light source, why does the current increase as you decrease the retarding potential, below the stopping potential. Isn't the current a measure of the number of electrons passing through the gap successfully? Below the stopping potential, shouldn't this depend only on the intensity?
Think of emitted electrons and the stopping potential as the equivalent of a bunch of bullets fired in all directions in a gravitational field. Depending on the direction a given bullet is fired in, it will reach a different height before it falls back to earth. If you find the maximum altitude the bullets reach, you can figure out the muzzle velocity of the guns. You could do this by flying an aircraft above the field, and listening for bullets hitting the belly. If you gradually fly higher and higher the number of impacts will gradually decrease, and when they stop altogether you've got the maximum altitude.
In the same way, electrons are emitted at varying angles, with different velocities perpendicular to the surface, and by gradually increasing the voltage you gradually select out the electrons with maximum velocity/energy. When the number of electrons detected (the current) drops to zero, you've got the maximum possible energy from the electrons.