The circuit board of your computer is chalk-full of electrical components and the circuits that run between them. Chances are you don’t know much about each little part, and most people don’t, but it’s safe to say that among your components are diodes, transistors, capacitors, resistors, and, one of the most interesting components of all: photoelectric cells.
What are photoelectric cells? They’re tiny devices that generate electricity when light falls on them. We really can’t go much further than that without a little lesson on photoelectricity:
Photoelectricity is simply electricity that is generated by a light beam. How does this process occur? Well, because light is a kind of electromagnetic energy, it can be thought of as a train of energetic particles called photons. The photons pass their energy in fixed quantities to atoms inside photoelectric cells, knocking around some of their electrons and thus producing an electric current. When Einstein explained this phenomenon in clear mathematical terms for the first time, he set the foundation for quantum theory and eventually won himself the Nobel Prize in Physics in 1921.
There are three different types of photoelectricity: photoconductive electricity, photoemissive electricity, and photovoltaic electricity.
Photoconductive electricity occurs when the presence of light decreases the resistance of a material, or makes it more conductive. This generally occurs because the electrons inside the material become more conductive when lights shines on them.
The photovoltaic effect is harnessed by solar panels, whether they be on calculators or digital watches. Similar to diodes, they’re made from two layers of semiconductive material placed one on top of the other. One has increased electrons (in comparison to its normal, insulator form) and one has decreased electrons (again, in comparison to its insulator form). When the electron-heavy layer is exposed to light, the electrons from the bottom layer travel up to the top layer, creating a current that can then be driven through an external circuit.
Photoemissive cells are constructed out of sealed glass vacuum tubes with a large metal plate (the negative terminal or cathode) along with a smaller, positively charged rod-like terminal (the anode). The cathode is made out of a light-sensitive material, so when photons hit it, electrons are forced to travel outward and become attracted to the positively-charged anode, which then channels them into a circuit and produces electric power with their movement.
All these different cells are used differently; photovoltaic cells are used to produce energy to be stored in batteries. Photoconductive cells are used as light detectors for things like bathroom faucets, intruder alarms, smoke detectors, automatically opening and closing doors, etc. They generally sense movement by having a beam of ultraviolet light constantly shining on them that acts as a source for an electric current. When an object moves between the light and the device, the resistance changes and the current weakens significantly, which is enough of a change to then trigger whatever service, be it turning on a faucet or opening a door.