Luminescence of light-emitting diode
At the moment when the circuit is connected, the voltage at both ends of C will not suddenly change, and the terminal voltage at the upper end is zero, so the triode cannot be connected, and the light-emitting diode does not emit light because there is no current. With the continuous charging of C, the terminal voltage of the upper terminal gradually rises, prompting the triode to start conducting, and then the LED starts to emit light.
The moment when the circuit is disconnected
At the moment when the circuit is disconnected, similarly, the voltage across C will not suddenly change. Since the triode is kept on, the current of the LED is kept at the maximum.
Emission of element c
With the continuous discharge of C, the voltage across C gradually decreases, and the current of LED also decreases. The transistor will not turn off until the voltage across C drops to a certain extent and the current of LED drops to zero.