photocell or photoelectric cell or electric eye Solid-state device with a photosensitive cathode that emits electrons when illuminated and an anode for collecting the emitted electrons. Illumination excites elec¬ trons, which are attracted to the anode, producing current proportional to the intensity of the illumination. In a photovoltaic cell, light is used to produce voltage. In a photoconductive cell, light is used to regulate the flow of current. Photocells are used in control systems, where interrupt¬ ing a beam of light opens a circuit, actuating a relay that supplies power to a mechanism to bring about a desired operation, such as opening a door or setting off a burglar alarm. Photocells are also used in photometry and SPECTROSCOPY.
photochemical reaction Chemical reaction initiated by absorption of energy in the form of visible (light), ultraviolet, or infrared radiation. Pri¬ mary photochemical processes occur as an immediate result, and second¬ ary processes may follow. The most important example is photosynthesis. Vision depends on photochemical reactions that occur in the eye (see retina; rhodopsin). In photographic film and paper, light activates their sil¬ ver salts to a state that is easy to reduce to metallic silver grains during development. Bleaching of laundry, tanning of human skin, formation of Earth’s ozone layer, and many industrial reactions are also photochemi¬
cal. Certain air pollutants (see smog) become more reactive and form noxious compounds in photochemical reactions.
photocopier Device for producing copies of text or graphic material by the use of light, heat, chemicals, or electrostatic charge. Most modern copiers use a method called xerography.
photoelectric effect Phenomenon in which charged particles are released from a material when it absorbs radiant energy (see radiation). It is often thought of as the ejection of electrons from the surface of a metal plate when visible light falls on it. It can also occur if the radiation is in the wave¬ length range of ultraviolet radiation, X rays, or gamma rays. The emitting sur¬ face may be a solid, liquid, or gas, and the emitted particles may be electrons or ions. The effect was discovered in 1887 by Heinrich Hertz and explained by Albert Einstein in work for which he received the Nobel Prize.
photoengraving Any of several processes for producing printing plates by photographic means (see photography). In general, a plate coated with a photosensitive substance is exposed to an image, usually on film; the plate is then treated in various ways, depending on the printing pro¬ cess to be used. Photoengraving is particularly useful for reproducing photographs via the halftone process. See also offset printing.
photography Method of recording permanent images by the action of light projected by a lens in a camera onto a film or other light-sensitive material. It was developed in the 19th century through the artistic aspi¬ rations of two Frenchmen, Nicephore Niepce and Louis-Jacques-Mande Daguerre, whose combined discoveries led to the invention of the first commercially successful process, the daguerreotype (1837). In addition, two Englishmen, Thomas Wedgwood and William Henry Fox Talbot, pat¬ ented the negative-positive calotype process (1839) that became the fore¬ runner of modern photographic technique. Photography was initially used for portraiture and landscapes. In the 1850s and ’60s, Mathew B. Brady and Roger Fenton pioneered war photography and photojournalism. From its inception, two views of photography predominated: one approach held that the camera and its resulting images truthfully document the real world, while the other considered the camera simply to be a tool, much like a paintbrush, with which to create artistic statements. The latter notion, known as Pictorialism, held sway from the late 1860s through the first decade of the 20th century, as photographers manipulated their nega¬ tives and prints to create hazy, elaborately staged images that resembled paintings. By the 1920s and ’30s, a new, more realistic style of photog¬ raphy gained prominence, as photographers such as Paul Strand, Edward Weston, and Ansel Adams began to pursue sharply focused, detailed images. The Great Depression and two world wars inspired many pho¬ tographers, including Walker Evans and Dorothea Lange, to pursue docu¬ mentary, often socially conscious photography. Inspired by such work, many photojournalists, including Alfred Eisenstaedt and Margaret Bourke- White, also emerged during this period. In the second half of the 20th century, the urban social scene became a subject of much interest to pho¬ tographers, as did celebrity portraiture and fashion photography. At the turn of the 21st century, photographers took advantage of digital capa¬ bilities by experimenting with enormous formats and new manipulative techniques. As technological advances improve photographic equipment, materials, and techniques, the scope of photography continues to expand enormously. See also digital camera.
photolysis \fo-'ta-l3-s9s\ Breakdown of molecules into smaller units via absorption of light. Flash photolysis, an experimental technique developed by Manfred Eigen, Ronald George Weyford Norrish, and George Porter, studies short-lived chemical intermediates formed in many photochemical reactions. An intense, brief flash of light splits molecules into short-lived fragments, which are analyzed by spectrophotometry in a second, less intense flash.
photometry \fo-'ta-m3-tre\ Precision measurement of the brightness, colour, and spectrum of stars and other celestial objects to obtain data on their structure, temperature, and composition. About 130 bc Hipparchus used a system that divided the stars into six magnitudes, from brightest to faintest. Beginning in the 17th century, use of the telescope led to the dis¬ covery of many fainter stars, and the scale was extended. The use of pho¬ tographic and, since the 1940s, photoelectric equipment has vastly extended the sensitivity and wavelength range of astronomical photom¬ etry. The main (UBVRI) classification system uses wave bands in the ultraviolet, blue, visual, red, and infrared ranges. More elaborate systems can distinguish giant and dwarf stars, detect metals in stars, and deter¬ mine surface gravity.
© 2006 Encyclopaedia Britannica, Inc.
photon ► Phraya Taksin I 1493
To Calvin cycle
or dark reaction
The light reaction of photosynthesis. The light reaction occurs in two photosystems (units of chlorophyll molecules). Light energy (indicated by wavy arrows) absorbed by photosystem II causes the formation of high-energy electrons, which are transferred along a series of acceptor molecules in an electron transport chain to photosystem I. Photosystem II obtains replacement elec¬ trons from water molecules, resulting in their split into hydrogen ions (H+) and oxygen atoms. The oxygen atoms combine to form molecular oxygen (0 2 ), which is released into the atmosphere. The hydrogen ions are released into the lumen. Additional hydrogen ions are pumped into the lumen by electron acceptor molecules. This creates a high concentration of ions inside the lumen. The flow of hydrogen ions back across the photosynthetic membrane provides the energy needed to drive the synthesis of the energy-rich molecule ATP. High-energy electrons released as photosystem I absorbs light energy are used to drive the syn¬ thesis of NADPH. Photosystem I obtains replacement electrons from the electron transport chain. ATP provides the energy and NADPH provides the hydrogen atoms needed to drive the subsequent photosynthetic dark reaction, or Calvin cycle.
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photon or light quantum Minute energy packet of electromagnetic radiation. In 1900 Max Planck found that heat radiation is emitted and absorbed in distinct units, which he called quanta. In 1905 Albert Einstein explained the photoelectric effect, proposing the existence of discrete energy packets in light. The term photon came into use for these packets in 1926. The energies of photons range from high-energy gamma rays and X rays to low-energy infrared and radio waves, though all travel at the same speed, the speed of light. Photons have no electric charge or rest mass and are the carriers of the electromagnetic field.