Have you ever wondered what is an LED?
here I can answer your question.
An LED belongs to family of diodes that emits light for which it is called as Light Emitting Diode.
FunctionLEDs emit light when an electric current passes through them.
Connecting and solderingLEDs must be connected the correct way round, the diagram may be labelled a or + for anode and k or - for cathode (yes, it really is k, not c, for cathode!). The cathode is the short lead and there may be a slight flat on the body of round LEDs. If you can see inside the LED the cathode is the larger electrode (but this is not an official identification method).
Testing an LEDNever connect an LED directly to a battery or power supply!
It will be destroyed almost instantly because too much current will pass through and burn it out.