EASA / CAR Part-66 · Module 4 · 4.1 Semiconductors · Cat B1/B2
An LED turns current into light; a photodiode turns light into current, opposites that pair up in optocouplers and sensors. Drive the LED and watch it light the photodiode across the gap.
The LED only conducts and emits above its forward voltage (about 1.8–2 V), and brightness rises with current. The photodiode receives that light and produces a proportional current, this is how an optocoupler passes a signal while keeping the two circuits electrically isolated.