5 mm Red LED with Difused Lens

The LED's are semiconductor devices that emit light when the current is being transmitted.

Those LEDS are commonly used for miniature timers, road stop signs and more. Its diffused lens gives a wider viewing angle of the red light.

LEDs do not allow reverse current flow if fed. With right current, they open after the voltage at the terminals exceeded a certain threshold value (allow the current to pass and light, the light intensity being proportional to the current). 

The opening threshold depends on the color of the LED, increasing from red (about 2.1 - 2.4 volts) to blue / white (about 3.2 - 3.8 volts)