Consider the circuit shown. You recognize a diode which is connected in the reverse direction. Behind it you find a Darlington pair.
Connecting a diode in the reverse direction blocks all current. However, this is only valid for an ideal diode.
For a real diodes a current considerably less than 0,1 μA flows in the reverse direction. This current is called a reverse current or leakage current.
The Darlington pair is able to amplify the leakage current enough to detect it. The LED1 lights up weakly!
The reverse current is dependent on many parameters, eg temperature. In the next experiment you examine this further...