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