Why is there a sudden increase in current in a Zener diode?

$A$ Zener diode with a Zener voltage of $30 \text{ V}$ is connected in a circuit as shown in the figure. The maximum current through the Zener diode is: (in $\text{ mA}$)

In a Zener diode,

The Zener diode is used for

$A$ potential divider circuit is connected to a $DC$ source of $20 \,V$, a light emitting diode $(LED)$ with a glow-in voltage of $1.8 \,V$, and a Zener diode with a breakdown voltage of $3.2 \,V$. The total length $(PR)$ of the resistive wire is $20 \,cm$. The minimum length of $PQ$ required for the $LED$ to just glow is ............. $cm$.

The correct symbol for a Zener diode is: