A potential divider circuit is connected with a dc source of 20 V, a light emitting diode of glow in voltage 1.8 V and a zener diode of breakdown…

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