When will a light emitting diode (LED) illuminate?

A light emitting diode (LED) illuminates when it is forward biased. In a forward-biased condition, the LED allows current to flow through it from the anode to the cathode. This flow of current excites the electrons in the semiconductor material of the LED, causing them to release energy in the form of photons, which we perceive as visible light. The specific energy bandgap of the semiconductor determines the color of the light emitted.

In contrast, when an LED is reverse biased, it does not conduct current under normal operating conditions, and therefore will not illuminate. If an LED is disconnected, there will be no current flowing through it, meaning it cannot produce light. Similarly, exposure to ambient light does not generate illumination in an LED, as LEDs require an electrical current to produce light through electroluminescence. Thus, the condition of being forward biased is essential for the LED to illuminate.