triggered the revolution in semiconductor electronics and physics. In materials with a direct band gap, if bright light is aimed at one part of the contact area between the two semiconductors, the voltage at that spot will rise, and an electric current will appear. When considering light in the context of contact electrification, the light energy is changed directly into electrical energy, allowing creation of solar cells. Later it was found that the same process can be reversed, and if a current is forced backwards across the contact region between the semiconductors, sometimes light will be emitted, allowing creation of the light-emitting diode (LED). References Electrical phenomena Obsolete theories in physics