The 3mm White-red light-emitting diode (LED) is a semiconductor light source. LED's are used as indicator lamps in many devices and are increasingly used for other lighting. It looks like a white led and illuminate's blue light. When a light-emitting diode is switched on, electrons are able to recombine with holes within the device, releasing energy in the form of photons. This effect is called electroluminescence and the colour of the light (corresponding to the energy of the photon) is determined by the energy bandgap of the semiconductor.