Light-emitting diodes (LEDs) are based on the semiconductor diode. When the diode is forward biased (switched on), electrons are able to recombine with holes and energy is released in the form of light. This effect is called electroluminescence. These blue LED's are from my binary clock, yes I know I'm a geek. Get your own binary clock at Think Geek. Canon 40D, 1/80 sec at f/5.0, ISO 100, Canon 85mm f/1.8 lens.