June 20th in clean tech history honors the death of German physicist Wilhelm Hallwachs (b. Darmstadt Germany, 9 July 1859; d. Dresden, Germany, 20 June 1922). As an experimental physicist, he laid the foundations for research on photoelectric processes and in 1904 discovered that a combination of copper and cuprous oxide is photosensitive. Modern photo cells are based off of his discoveries.
At Leipzig in 1888, Hallwachs investigated photoelectric activity. He followed the model of Heinrich Hertz’s studies, of whom he was a student. Hallwachs established that through absorption of ultraviolet light, negatively charged metal plates discharge and uncharged metal plates become positively charged.
This process, which is called the photoelectric effect or Hallwachs effect. The photoelectric effect is a phenomenon in which electrons are emitted from matter (metals and non-metallic solids, liquids or gases) as a consequence of their absorption of energy from electromagnetic radiation of very short wavelength, such as visible or ultraviolet light. Electrons emitted in this manner may be referred to as "photoelectrons."
The photoelectric effect forms the basis for the physics of the photoelectric cell and was theoretically interpreted in 1905 in Einstein’s work on light quanta. Hallwachs’ observations laid the foundation for the later development of photo cells, TV camera imaging and other light-sensitive electronic devices.