Between the first anode and the second anode there is a slight pot dif. The slow electrons don't get to the 2nd anode, which is hooked up to an ammeter. A big drop is found at 4.9 V, 6.7V, 8.8V, and 9.8 V Before the pot dif hits 4.9V, the electrons flying through don't have enough energy to give up to the Hg. When they hit 4.9, they have the right amount of energy to give. Cathode is a source of electrons.
Different metals have different threshold frequencies.
Light is incident on the cathode. For a lower intensity light, the current still drops to 0 at the same place. This is called the stopping potential. A different frequency of light will have a different stopping potential. Each electron is assumed to only be able to absorb a quantum of energy.