The conversion of photons into electrons. When light is beamed onto a metal, electrons are released from its atoms. The higher the light frequency, the more electron energy released. Photonic sensors of all kinds work on this principle. They sense light and cause an electric current to flow.
Although many people think Einstein won the Nobel Prize because of his Theory of Relativity, it was actually due to his discovery of the photoelectric effect. He theorized that light was made of particles (later called photons) and that it carried an amount of energy exactly proportional to its frequency. See phototransistor
A Photoelectric Sensor
The red arrow points to the light sensor, while the blue arrow points to the single LED in this night light. See LED bulb