In this experiment, you use no antenna, but a LED connected in reverse direction. In the last experiment, you got to know the reverse current of a diode. A LED displays the same behavior. However, the reverse current of a LED is strongly dependent on the incident light. Therefore, you can use a LED as a light detector.
Consider the circuit shown. You see the yellow light emitting diode LED1, which is connected in reverse direction. Behind it you find the Darlington pair.
The LED2 lights up when you shine a flashlight on LED1.
The yellow LED has become a light detector!