Hint: Recall the concept of a Zener diode and a solar cell.
Step 1: Find the type of curves.
(i) The characteristic curve (a) is of Zener diode and curve (b) is of the solar cell.
Step 2: Find the nature of point P in curve (a).
(ii) The point P in fig. (a) represents Zener breakdown voltage.
Step 3: Find the nature of point P and Q in curve (b).
(iii) In fig. (b), the point Q represents zero voltage and negative current. It means light falling on the solar cell with at least minimum threshold frequency gives the current in opposite direction to that due to a battery connected to the solar cell. But for point Q, the battery is short-circuited. Hence represents the short circuit current.
In fig. (b), point P represents some positive voltage on the solar cell with zero current through the solar cell.
It means, there is a battery connected to a solar cell which gives rise to the equal and opposite current to that in the solar cell by virtue of light falling on it.
As current is zero for point P, hence we say P represents open-circuit voltage.
