Q.The LED emits visible light when its ______.

The Light Emitting Diode (LED) is a p-n junction diode, with an n-region doped more heavily than its p-region, that emits light when a sufficiently large forward current passes through it. Its junction is encased in a dome-shaped transparent case that lets light escape uniformly in all directions while minimising internal reflection losses, with the longer of its two leads conventionally being the anode (positive terminal). When forward biased, electrons injected from the n-side recombine with holes near the junction; each recombination event releases its excess energy -- approximately equal to the material's band gap $E_g$ -- as an emitted PHOTON rather than as heat, in a suitably chosen (typically direct-band-gap) compound semiconductor. Since this energy is fairly well-defined, the emitted light is essentially monochromatic, with its specific colour/wavelength set by the material's band gap. …