Bono: …finally subjugated in 1722–23 by Opoku Ware of the Asante empire.

Opole: …the home of the Slavic Opolanie tribe; the earliest mention of it was in the 9th century. In 1202 it became the capital of the Opole principality, which included the entire Upper Silesia region. The town passed to Bohemia (1327), the Habsburgs (16th century), and Prussia (1742) and was returned…

Opole, city, capital of Opolskie województwo (province), southwestern Poland, situated on the Oder River. Opole began as the home of the Slavic Opolanie tribe; the earliest mention of it was in the 9th century. In 1202 it became the capital of the Opole principality, which included the entire Upper

Opole: …became the capital of the Opole principality, which included the entire Upper Silesia region. The town passed to Bohemia (1327), the Habsburgs (16th century), and Prussia (1742) and was returned to Poland in 1945.

Opolskie: History: …the duchy of Opole-Racibórz (Upper Silesia). During the rule of the “Silesian Henries” (1202–41), a number of Germans settled in the duchy of Opole-Racibórz. It was a time of economic prosperity, and Opole, Nysa, and Brzeg developed as trade centres. In the 14th century the duchy of Opole-Racibórz split…

Opolskie, województwo (province), southern Poland. It is bordered by the provinces of Wielkopolskie and Łódzkie to the north and Śląskie to the east, by the Czech Republic to the south, and by the province of Dolnośląskie to the west. Created as one of Poland’s 16 reorganized provinces in 1999, it

Monique Wittig: Her first novel, L’Opoponax (1964; The Opoponax), is an examination of childhood experiences viewed through the consciousness of a rebellious young girl in a convent school. Its unorthodox, minimally punctuated, and nonchronological narrative established Wittig’s course as a writer. She sought to avoid traditional forms and accepted devices, the use…

Monique Wittig: Her first novel, L’Opoponax (1964; The Opoponax), is an examination of childhood experiences viewed through the consciousness of a rebellious young girl in a convent school. Its unorthodox, minimally punctuated, and nonchronological narrative established Wittig’s course as a writer. She sought to avoid traditional forms and accepted devices, the use…

Porto, city and port, northern Portugal. The city lies along the Douro River, 2 miles (3 km) from the river’s mouth on the Atlantic Ocean and 175 miles (280 km) north of Lisbon. World-famous for its port wine, Porto is Portugal’s second largest city and is the commercial and industrial centre for

opossum, any of slightly more than 100 species of New World marsupial mammals in the orders Didelphimorphia, Paucituberculata (see rat opossum), and Microbiotheria (see monito del monte). These marsupials, along with their relatives in Australasia, were formerly grouped together in the order

marsupial: Classification: Family Didelphidae (American opossums)70 or more species in 12 genera. Order Paucituberculata (shrew, or rat, opossums)5 species in 1 family.Family Caenolestidae5 species in 3 genera.

opossum shrimp, any member of the crustacean order Mysidacea. Most of the nearly 1,000 known species live in the sea; a few live in brackish water; and fewer still live in fresh water. Most are 1 to 3 cm (about 0.4 to 1.2 inches) long. The name opossum shrimp derives from the females’ brood pouch,

Lepidoptera: Annotated classification: Family Opostegidae Approximately 100 worldwide species of small moths with narrow long-fringed wings; larvae leaf, stem, or bark miners. Superfamily Tischerioidea Approximately 80 species in a single family. Family Tischeriidae (trumpet leaf miner moths)

Opus, in ancient Greece, the chief city of the Locri Opuntii. Its site may have been at modern Atalándi or at Kiparíssi. Homer in his Iliad mentioned Opus, and Pindar devoted his ninth Olympian ode mainly to its glory and traditions. By the 5th century bc, Opus gave its name to some of the eastern

Olga Tokarczuk: Novels and other works: …Lost Soul), a children’s book; Opowiadania bizarne (2018; “Tales of the Bizarre”), a book of short stories; and Czuły narrator (2020; “The Tender Narrator”), a collection of essays and lectures. In 2024 she published Empuzjon (The Empusium), her first novel since winning the Nobel Prize. Set in a Polish sanitarium…

Viktor Shklovsky: Petersburg, Shklovsky helped found OPOYAZ, the Society for the Study of Poetic Language, in 1914. He was also connected with the Serapion Brothers, a collection of writers that began meeting in Petrograd (St. Petersburg) in 1921. Both groups felt that literature’s importance lay primarily not in its social content…

Albert Oppel was a German geologist and paleontologist, who was one of the most important early stratigraphers. Oppel was a professor at Munich from 1861. In studying the Swabian Jura he discovered that paleontologic and lithologic zones need not be identical or even mutually dependent. His use of

Opole, city, capital of Opolskie województwo (province), southwestern Poland, situated on the Oder River. Opole began as the home of the Slavic Opolanie tribe; the earliest mention of it was in the 9th century. In 1202 it became the capital of the Opole principality, which included the entire Upper

George Oppen was an American poet and political activist, one of the chief proponents of Objectivism, a variation on Imagism. Oppen grew up in San Francisco and briefly attended Oregon State University, where he met his wife. In 1929 the Oppens moved to Paris, where from 1930 to 1933 they ran the

E. Phillips Oppenheim was an internationally popular British author of novels and short stories dealing with international espionage and intrigue. After leaving school at age 17 to help in his father’s leather business, Oppenheim wrote in his spare time. His first novel, Expiation (1886), and

E. Phillips Oppenheim was an internationally popular British author of novels and short stories dealing with international espionage and intrigue. After leaving school at age 17 to help in his father’s leather business, Oppenheim wrote in his spare time. His first novel, Expiation (1886), and

Lassa Francis Lawrence Oppenheim was a German jurist and teacher of law who was best known for his Positivist approach to international law. Oppenheim moved from Basel, Switz., to London, where he joined the faculty of the newly organized London School of Economics and Political Science in 1895. In

Meret Oppenheim was a German-born Swiss artist whose Object (1936), a fur-covered teacup, saucer, and spoon, became an emblem of the Surrealist movement. The piece, created when Oppenheim was just 23 years old, became so famous that it overshadowed the rest of her career. Oppenheim’s father was

Meret Oppenheim was a German-born Swiss artist whose Object (1936), a fur-covered teacup, saucer, and spoon, became an emblem of the Surrealist movement. The piece, created when Oppenheim was just 23 years old, became so famous that it overshadowed the rest of her career. Oppenheim’s father was

The true story of Oppenheimer and the atomic bombJ. Robert Oppenheimer became involved in nuclear research in 1941. His biopic, Oppenheimer, was released in 2023.Encyclopædia Britannica, Inc.; thumbnail © Universal Pictures Oppenheimer, American and British dramatic biographical film, released in

J. Robert Oppenheimer was an American theoretical physicist and science administrator, noted as director of the Los Alamos Laboratory (1943–45) during development of the atomic bomb and as director of the Institute for Advanced Study, Princeton (1947–66). Accusations of disloyalty led to a

J. Robert Oppenheimer was an American theoretical physicist and science administrator, noted as director of the Los Alamos Laboratory (1943–45) during development of the atomic bomb and as director of the Institute for Advanced Study, Princeton (1947–66). Accusations of disloyalty led to a

Katherine Oppenheimer was a German American botanist, biologist, and wife of Los Alamos Laboratory director J. Robert Oppenheimer. After her birth in Germany, Katherine Puening moved with her family to Pittsburgh, Pennsylvania, at age three and spent the rest of her childhood in the area. Her

Katherine Oppenheimer was a German American botanist, biologist, and wife of Los Alamos Laboratory director J. Robert Oppenheimer. After her birth in Germany, Katherine Puening moved with her family to Pittsburgh, Pennsylvania, at age three and spent the rest of her childhood in the area. Her

Max Ophüls was a German motion-picture director whose mastery of fluid camera movement gave his films a characteristic lyrical flow. He was one of the first truly international directors, sensitive to national differences and to the human qualities common to all his characters. Ophüls was an actor,

Austria: Social, economic, and cultural trends in the Baroque period: …to rich bankers such as Samuel Oppenheimer and his successor Samson Wertheimer for funds. Soon, however, it attempted to establish state-controlled banking firms. The Banco del Giro, founded in Vienna in 1703, quickly failed, but the Vienna Stadtbanco of 1705 managed to survive; the Universalbancalität of 1715 was liquidated after…

Sir Ernest Oppenheimer was a German-born industrialist, financier, and one of the most successful leaders in the mining industry in South Africa and Rhodesia. Oppenheimer became a junior clerk at the age of 16 with Dunkelsbuhlers & Company, London diamond brokers. In 1902 he moved to Kimberley,

Western architecture: France: …France from Russia, he, with Gilles-Marie Oppenordt and Juste-Aurèle Meissonier, who were increasingly concerned with asymmetry, created the full Rococo. Meissonier and Oppenordt should be noted too for their exquisite, imaginative architectural designs that were unfortunately never built (e.g., facade of Saint-Sulpice, Paris, 1726, by Meissonier).

South African literature: In Afrikaans: The poet D.J. Opperman came into prominence in 1945. His technique superimposes different historical levels intermingled with a fascinating mosaic of themes, images, and allusions from both Africa and a common Western heritage. Ernst van Heerden, another major poet who emerged in the 1940s, veered from tightly…

dress: Sumptuary laws: …under the Roman Republic, the Lex Oppia, was enacted in 215 bce; it ruled that women could not wear more than half an ounce of gold upon their persons and that their tunics should not be in different colours. Most Roman sumptuary laws tried to control spending on funerals, banquets,…

dress: Sumptuary laws: …under the Roman Republic, the Lex Oppia, was enacted in 215 bce; it ruled that women could not wear more than half an ounce of gold upon their persons and that their tunics should not be in different colours. Most Roman sumptuary laws tried to control spending on funerals, banquets,…

history of Europe: Prestige and status: …particularly strongly suggested by the oppida of western, central, and eastern Europe. These were often densely populated enclosed sites, which housed full-time specialists, such as glassmakers, leather workers, and smiths. Manching, one of the largest oppida in Europe, contained many of these characteristics. The site, located at the junction of…

Eton College: The other students, called Oppidans, now number more than 1,200 and are housed in boardinghouses under the care of house masters. The Oppidans have traditionally come from England’s wealthiest and most prestigious families, many of them aristocratic. Boys enter Eton about age 13 and continue there until they are…

Girona, city, capital of Girona provincia (province), in the Catalonia comunidad autónoma (autonomous community), northeastern Spain. It lies on the Oñar River in the foothills of the Los Ángeles Mountains, a short distance inland from a Mediterranean coastal resort area known as the Costa Brava.

opportunism, a foundational assumption of many economic theories that claims human beings are generally self-interested and will take advantage of others when possible. For example, some economic actors will take advantage of another party to advance their own interests by making false promises,

France: Republican factions: …1880s the labels Radical and Opportunist began to be attached to the two wings of the republican movement. On the left, the Radicals saw themselves as heirs to the Jacobin tradition: they stood for a strong centralized regime, intransigent anticlericalism, an assertive nationalism in foreign policy, a revision of the…

cesarean section: Risks: …are a common cause of opportunistic infections in hospital environments. Although, over time, the microbiome in affected infants becomes more similar to that in infants delivered vaginally, early differences in the microbiome may impact infant health or contribute to health issues that arise later.

Mars: Spacecraft exploration: …twin robotic landers, Spirit and Opportunity. Spirit touched down in Gusev Crater (15° S 175° E) on January 3, 2004. Three weeks later, on January 24, Opportunity landed in Meridiani Planum (2° S 6° W), on the opposite side of the planet. The six-wheeled rovers, each equipped with cameras and…

Opportunity, American magazine associated with the Harlem Renaissance, published from 1923 to 1949. The editor, Charles S. Johnson, aimed to give voice to black culture, hitherto neglected by mainstream American publishing. To encourage young writers to submit their work, Johnson sponsored three

opportunity cost, In economic terms, the opportunities forgone in the choice of one expenditure over others. For a consumer with a fixed income, the opportunity cost of buying a new dishwasher might be the value of a vacation trip never taken or several suits of clothes unbought. The concept of

Opportunity, American magazine associated with the Harlem Renaissance, published from 1923 to 1949. The editor, Charles S. Johnson, aimed to give voice to black culture, hitherto neglected by mainstream American publishing. To encourage young writers to submit their work, Johnson sponsored three

primate: …lemurs and lorises have an opposable thumb. Primates are not alone in having grasping feet, but as these occur in many other arboreal mammals (e.g., squirrels and opossums), and as most present-day primates are arboreal, this characteristic suggests that they evolved from an ancestor that was arboreal. So too does…

Johann Georg Bodmer: …with inventing the cylinder with opposed pistons.

Johann Georg Bodmer: …with inventing the cylinder with opposed pistons.

angiosperm: Leaves: In opposite-leaved plants, the leaves are paired at a node and borne opposite to each other. A plant has whorled leaves when there are three or more equally spaced leaves at a node.

Lisa Kudrow: Career and personal life: … (1997), the dark romantic comedy The Opposite of Sex (1998), the mafia comedy Analyze This (1999) and its sequel Analyze That (2002), and the crime drama Wonderland (2003). She also voiced the roles of Aphrodite in the animated TV series Hercules (1998–99) and Ava the Pacific western bear in the…

table of opposites, in Pythagorean philosophy, a set of 10 pairs of contrary qualities. The earliest reference is in Aristotle, who said that it was in use among some contemporary Pythagoreans. But Aristotle provided no real information about its function in Pythagorean practice or theory or about

joint: Sellar joint: This movement is called opposition (i.e., of thumb to fingers). During opposition the thumb is rotated around its long axis; it has been said that human civilization depends upon the opposition of the thumb.

opposition, in astronomy, the circumstance in which two celestial bodies appear in opposite directions in the sky. The Moon, when full, is said to be in opposition to the Sun; the Earth is then approximately between them. A superior planet (one with an orbit farther from the Sun than Earth’s) is in

United Kingdom: The supremacy of the Whigs: …and began to flirt with Opposition groups in Parliament. These events set the pattern for future political conflicts. From then on until the 1750s the Opposition in Parliament would be a hybrid group of Whig and Tory sympathizers. And from then on until the early 19th century Oppositions in Parliament…

Uranus: Moons of Uranus: Such so-called opposition surges are characteristic of loosely stacked particles that shadow each other except in this special geometry, in which the observer is in line with the source of illumination and can see the light reflecting directly back out of the spaces between the particles. Second,…

square of opposition, in traditional logic, a diagram exhibiting four forms of a categorical proposition (q.v.), or statement, with the same subject and predicate, together with their pairwise relationships:in which A, E, I, and O are of the forms “Every S is P,” “No S is P,” “Some S is P,” and

Czechoslovak history: Struggle for independence: …anti-Austrian resolution adopted at the Congress of Oppressed Nationalities, held in Rome in April, helped to disarm conservative circles in Allied countries that had opposed a total reorganization of the Danubian region. Eventually, France recognized the Czechoslovak National Council as the supreme body controlling Czechoslovak national interests; the other Allies…

Augusto Boal: …Brazilian dramatist who created the Theatre of the Oppressed, a form of interactive theatre intended to transform lives as spectators become performers, acting out solutions to social problems.

crime: Interrogation and confession: …it was not obtained by oppression of the person who made it (e.g., by torture, inhuman or degrading treatment, the use or threat of violence, or excessively prolonged periods of questioning) or as a result of anything said or done that would be likely to render the confession unreliable.

Mehmet Oz: Medical career: …to a television appearance on The Oprah Winfrey Show. Oz subsequently became a regular guest on that program as well as many others, earning him the nickname “America’s Doctor.” His rapport with audiences was in part due to his easygoing manner and holistic approach to health. With Roizen he continued…

Oprah Winfrey: In 2006 the Oprah & Friends channel debuted on satellite radio. She brokered a partnership with Discovery Communications in 2008, through which the Oprah Winfrey Network (OWN) replaced the Discovery Health Channel in January 2011. In 2009 Winfrey announced that her television talk show would end in 2011;…

Oprah Winfrey: …she opened a $40 million school for disadvantaged girls in South Africa. She became an outspoken crusader against child abuse and received many honours and awards from civic, philanthropic, and entertainment organizations. In 2010 she was named a Kennedy Center honoree, and the following year she received the Jean Hersholt…

Tyler Perry: …picked up by Oprah Winfrey’s OWN channel in its third season following its cancellation by TBS. Perry created several additional television shows for OWN: the soap operas The Haves and the Have Nots (2013–21) and If Loving You Is Wrong (2014–16) as well as such comedies as Love Thy Neighbor…

Mehmet Oz: Medical career: …to a television appearance on The Oprah Winfrey Show. Oz subsequently became a regular guest on that program as well as many others, earning him the nickname “America’s Doctor.” His rapport with audiences was in part due to his easygoing manner and holistic approach to health. With Roizen he continued…

Pyotr Ilyich Tchaikovsky: Middle years: …1872 he finished another opera, The Oprichnik. While spending the summer at his sister’s estate in Ukraine, he began to work on his Symphony No. 2 in C Minor, later dubbed The Little Russian, which he completed later that year. The Oprichnik was first performed at the Maryinsky Theatre in…

oprichnina, private court or household created by Tsar Ivan IV the Terrible (1565) that administered those Russian lands (also known as oprichnina) that had been separated from the rest of Muscovy and placed under the tsar’s direct control. The term also refers generally to the economic and

Ops, Roman goddess (originally perhaps of the Earth’s fertility) with an ancient shrine in the Regia, the office of the pontifex maximus, which only he and the Vestal Virgins might enter. She was early equated with Rhea, wife of Saturn, and like her was later identified with Cybele. She also had

paracanthopterygian: Life cycle and reproduction: Eggs of the oyster toadfish (Opsanus tau) of the western Atlantic—one of the most carefully studied batrachoidiforms—are laid in dark recesses of all sorts, including sunken tin cans and shoes. The male guards the eggs and young for about three weeks, after which the young begin life on…

coloration: Visual functions: …that consists of a protein, opsin, attached to a chromophore. The chromophore may be either retinal (vitamin A1), in which case the molecule is called rhodopsin; or 3-dehydroretinal (vitamin A2), in which case the molecule is called porphyropsin. When light enters the eye and strikes the visual biochrome, the molecule…

immune system: Activation of the complement system: …pathogen in a process called opsonization. This makes the microorganism more attractive to phagocytic cells such as macrophages and neutrophils. The attraction occurs because receptors on the surface of phagocytes recognize and bind to the C3b molecule on the surface of the pathogen, stimulating phagocytosis. The microbe is then killed…

immune system: Activation of the complement system: …pathogen in a process called opsonization. This makes the microorganism more attractive to phagocytic cells such as macrophages and neutrophils. The attraction occurs because receptors on the surface of phagocytes recognize and bind to the C3b molecule on the surface of the pathogen, stimulating phagocytosis. The microbe is then killed…

Jo van Ammers-Küller: …successful novel, De opstandigen (1925; The Rebel Generation), presents the struggle of three generations of women in the Coornvelt family for equality with men and against the strictures of their Calvinist environment.

patristic literature: The post-Nicene Latin Fathers: …the measured anti-Donatist polemic of Optatus of Milevis, writing in 366 or 367, whose line of argument anticipates Augustine’s later attack against the Donatists.

Gordon Gould: …founded an optical communications company, Optelecom, in 1973. He retired from Optelecom in 1985, and he was inducted into the (U.S.) National Inventors Hall of Fame in 1991.

Advanced Micro Devices, Inc.: …2003 the company released the Opteron chip, another product that showcased the company’s ability to produce high-end chips. In 2006 AMD absorbed ATI Technologies, a manufacturer of video graphics cards for use in PCs. In 2008 AMD announced plans to split the company in two—with one part designing microprocessors and…

optic ataxia, condition in which some or all aspects of visual guidance over reaching with the hand and arm are lost. Optic ataxia is broadly characterized by an inaccuracy of visually guided arm movements. In reaching for an object, a person with severe optic ataxia may seem to grope in the dark,

optic atrophy, degeneration of the optic nerve (the second cranial nerve) due to direct or indirect damage to a particular type of retinal cell, called ganglion cells, whose axonal projections collectively make up the optic nerve. The function of the optic nerve is to carry visual data from the

double refraction: …along the direction of its optic axis, however, the light ray will not become divided.

human eye: The visual pathway: The optic nerves after this point are called the optic tracts, containing nerve fibres from both retinas. The result of the partial decussation is that an object in, say, the right-hand visual field produces effects in the two eyes that are transmitted to the left-hand side…

human eye: The visual pathway: The optic nerves after this point are called the optic tracts, containing nerve fibres from both retinas. The result of the partial decussation is that an object in, say, the right-hand visual field produces effects in the two eyes that are transmitted to the left-hand side…

animal development: The eye: …the vesicle becomes a double-walled optic cup. The thick inner layer of the optic cup gives rise to the sensory retina of the eye; the thinner outer layer becomes the pigment coat of the retina. The opening of the optic cup, wide at first, gradually becomes constricted to form the…

blind spot, small portion of the visual field of each eye that corresponds to the position of the optic disk (also known as the optic nerve head) within the retina. There are no photoreceptors (i.e., rods or cones) in the optic disk, and, therefore, there is no image detection in this area. The

human eye: The orbit: The optic foramen, the opening through which the optic nerve runs back into the brain and the large ophthalmic artery enters the orbit, is at the nasal side of the apex; the superior orbital fissure is a larger hole through which pass large veins and nerves.…

endocrine system: Phylum Mollusca: …information detected by the so-called optic gland (located near the eye) can direct the release of the gonadotropic hormone. The gonadotropic hormones that cause egg laying in Aplysia and Lymnaea have been isolated, and they are very similar small peptides. The hermaphroditic gonad of Euhadra secretes testosterone (identical to the…

glioma: …different types of gliomas include optic glioma, which affects the optic nerve in the brain; oligodendroglial tumours, which originate with oligodendrocytes, a type of neuroglia that produces myelin (the insulating sheath on the axons of nerves); and ependymomas, which originate with ependymal cells, a

nervous system: Encephalization: The optic lobes, especially prominent in fish and birds, are a part of this area. In fish and amphibians the tectum is the major centre of the nervous system and wields the greatest influence on body activity. While this area is still significant in reptiles and…

optic nerve, second cranial nerve, which carries sensory nerve impulses from the more than one million ganglion cells of the retina toward the visual centres in the brain. The vast majority of optic nerve fibres convey information regarding central vision. The optic nerve begins at the optic disk,

glioma: …different types of gliomas include optic glioma, which affects the optic nerve in the brain; oligodendroglial tumours, which originate with oligodendrocytes, a type of neuroglia that produces myelin (the insulating sheath on the axons of nerves); and ependymomas, which originate with ependymal cells, a

optic neuritis, inflammation of the optic nerve (the second cranial nerve). The inflammation causes a fairly rapid loss of vision in the affected eye, a new blind spot (a scotoma, usually in or near the centre of the visual field), pain in the eyeball (often occurring with eye movement), abnormal

human nervous system: Optic nerve (CN II or 2): In this way the optic tracts, which extend from the chiasm to the thalamus, contain fibers conveying information from both eyes. Injury to one optic nerve therefore results in total blindness of that eye, while damage to the optic tract on one side results in partial blindness in both…

animal development: The eye: …of the eyes develop from optic vesicles, each of which remains connected to the brain by an eye stalk, which later serves as the pathway for the optic nerve. The optic vesicles extend laterally until they reach the skin, whereupon the outer surface caves in so that the vesicle becomes…

William Taylor Adams was an American teacher and author of juvenile literature, best known for his children’s magazine and the series of adventure books that he wrote under his pseudonym. Although he never graduated from college, Adams was a teacher and principal in Boston elementary schools for

Ptolemy: Mathematician: …study of visual perception in Optica (“Optics”), a work that only survives in a mutilated medieval Latin translation of an Arabic translation. The extent to which Ptolemy subjected visual perception to empirical analysis is remarkable when contrasted with other Greek writers on optics. For example, Hero of Alexandria (mid-1st century…

James Gregory: …to London where he published Optica Promota (1663; “The Advance of Optics”). This work analyzed the refractive and reflective properties of lens and mirrors based on various conic sections and substantially developed Johannes Kepler’s theory of the telescope. In the epilogue, Gregory proposed a new telescope design with a secondary…

optical activity, the ability of a substance to rotate the plane of polarization of a beam of light that is passed through it. (In plane-polarized light, the vibrations of the electric field are confined to a single plane.) The intensity of optical activity is expressed in terms of a quantity,

laser: Laser elements: …laser would just be an optical amplifier, which can amplify light from an external source but not generate a beam internally. Elias Snitzer, a researcher at American Optical, demonstrated the first optical amplifier in 1961, but such devices were little used until the spread of communications based on fibre optics.

enantiomer, either of a pair of objects related to each other as the right hand is to the left—that is, as mirror images that cannot be reoriented so as to appear identical. An object that has a plane of symmetry cannot be an enantiomer because the object and its mirror image are identical.

Op art, branch of mid-20th-century geometric abstract art that deals with optical illusion. Achieved through the systematic and precise manipulation of shapes and colors, the effects of Op art can be based either on perspective illusion or on chromatic tension; in painting, the dominant medium of

optical axis, the straight line passing through the geometrical centre of a lens and joining the two centres of curvature of its surfaces. Sometimes the optical axis of a lens is called its principal axis. The path of a light ray along this axis is perpendicular to the surfaces and, as such, will