A knuckleball or knuckler is a baseball pitch thrown to minimize the spin of the ball in flight, causing an erratic, unpredictable motion. The air flow over a seam of the ball causes the ball to transition from laminar to turbulent flow; this transition adds a deflecting force on the side of the baseball. This makes the pitch difficult for batters to hit, but difficult for pitchers to control and catchers to catch. A pitcher who throws knuckleballs is known as a knuckleballer; the origins of the knuckleball are unclear. Toad Ramsey of the Louisville Colonels in the old American Association—his pitch resembled the knuckle curve—and Eddie Cicotte of the major leagues' Chicago White Sox, who in 1908, was nicknamed "Knuckles", are two possible creators of the pitch. Other accounts attribute the pitch's creation to Charles H. Druery, a pitcher for the Blue Ridge League. In 1917, Druery taught the pitch to Eddie Rommel who became successful with the knuckleball for the Philadelphia Athletics; as used by Cicotte, the knuckleball was thrown by holding the ball with the knuckles, hence the name of the pitch.
Ed Summers, a Pittsburgh teammate of Cicotte who adopted the pitch and helped develop it, modified this by holding the ball with his fingertips and using the thumb for balance. This grip can include digging the fingernails into the surface of the ball; the fingertip grip is more used today by knuckleball pitchers, like retired Boston Red Sox pitcher Tim Wakefield, who had a knuckleball with a lot of movement. There are other prominent knuckleball pitchers like Hall of Famer Phil Niekro, who had a effective knuckler and knuckle curve, Cy Young Award winning pitcher R. A. Dickey. However, young pitchers with smaller hands tend to throw the knuckleball with their knuckles. Sometimes young players will throw the knuckleball with their knuckles flat against the ball, giving it less spin but making it difficult to throw any significant distance. Regardless of how the ball is gripped, the purpose of the knuckleball is to have the least possible amount of rotational spin. Created by the act of throwing a ball, the ball's trajectory is affected by variations in airflow caused by differences between the smooth surface of the ball and the stitching of its seams.
The asymmetric drag. Over the distance from the pitcher's mound to home plate, the effect of these forces is that the knuckleball can "flutter," "dance," "jiggle," or curve in two different directions during its flight. A pitch thrown without spin is less desirable, than one with only a slight spin; this will cause the position of the stitches to change as the ball travels, which changes the drag that gives the ball its motion, thus making its flight more erratic. A ball thrown without rotation will "flutter", due to the "apparent wind" it feels as its trajectory changes throughout its flight path. Hitting a knuckleball is different enough from other aspects of baseball that players prepare for the pitch during batting practice before games they expect it in. According to physicist Robert Adair, due to the physiological limitation of human reaction time, a breaking knuckleball may be impossible to hit except by luck. If a knuckleball does not change direction in mid-flight, however it is easy to hit due to its lack of speed.
Since it only travels 60 to 70 miles per hour, far slower than the average major league fastball 85 to 95 miles per hour, it can be hit hard if there is no movement. One 2007 study offered evidence for this conclusion. To reduce the chances of having the knuckleball get hit for a home run, some pitchers will impart a slight topspin so that if no force causes the ball to dance, it will move downward in flight. Another drawback is that runners on base can advance more than if a conventional pitcher is on the mound; this is due to both the knuckleball's low average speed and its erratic movement, which force the catcher to keep focusing on the ball after the runners start stealing their next bases. However, since a typical major league starting rotation exceeds the length of a series against any one opponent, one way a manager can mitigate this disadvantage is to adjust his team's pitching rotation so as to eliminate games in which a knuckleballer would pitch against teams with a preponderance of fast baserunners.
Some knuckleball pitchers, such as Hoyt Wilhelm and Tim Wakefield, had catchers assigned to them to catch their knuckleballs. A paper presented at the 2012 Conference of the International Sports Engineering Association argues, based on PITCHf/x data, that knuckleballs do not make large and abrupt changes in their trajectories on the way to home plate—or at least, no more abrupt than a normal pitch, it speculates that the appearance of abrupt shifting may be due to the unpredictability of the changes in direction. The knuckleball are employed by cricket fast bowlers Andrew Tye, Bhuvneshwar Kumar and Zaheer Khan as their slower delivery; the physics of the operation are the same. However, the seam on a cricket ball is equatorial, thus the extent of erratic movement is reduced due to the symmetry (at least in the conventional release position whe
Major League Baseball
Major League Baseball is a professional baseball organization, the oldest of the four major professional sports leagues in the United States and Canada. A total of 30 teams play with 15 teams in each league; the NL and AL were formed as separate legal entities in 1901 respectively. After cooperating but remaining separate entities beginning in 1903, the leagues merged into a single organization led by the Commissioner of Baseball in 2000; the organization oversees Minor League Baseball, which comprises 256 teams affiliated with the Major League clubs. With the World Baseball Softball Confederation, MLB manages the international World Baseball Classic tournament. Baseball's first all-professional team was founded in Cincinnati in 1869; the first few decades of professional baseball were characterized by rivalries between leagues and by players who jumped from one team or league to another. The period before 1920 in baseball was known as the dead-ball era. Baseball survived a conspiracy to fix the 1919 World Series, which came to be known as the Black Sox Scandal.
The sport rose in popularity in the 1920s, survived potential downturns during the Great Depression and World War II. Shortly after the war, Jackie Robinson broke baseball's color barrier; the 1950s and 1960s were a time of expansion for the AL and NL new stadiums and artificial turf surfaces began to change the game in the 1970s and 1980s. Home runs dominated the game during the 1990s, media reports began to discuss the use of anabolic steroids among Major League players in the mid-2000s. In 2006, an investigation produced the Mitchell Report, which implicated many players in the use of performance-enhancing substances, including at least one player from each team. Today, MLB is composed of 1 in Canada. Teams play 162 games each season and five teams in each league advance to a four-round postseason tournament that culminates in the World Series, a best-of-seven championship series between the two league champions that dates to 1903. Baseball broadcasts are aired on television and the Internet throughout North America and in several other countries throughout the world.
MLB has the highest season attendance of any sports league in the world with more than 73 million spectators in 2015. MLB is governed by the Major League Baseball Constitution; this document has undergone several incarnations since its creation in 1876. Under the direction of the Commissioner of Baseball, MLB hires and maintains the sport's umpiring crews, negotiates marketing and television contracts. MLB maintains a unique, controlling relationship over the sport, including most aspects of Minor League Baseball; this is due in large part to the 1922 U. S. Supreme Court ruling in Federal Baseball Club v. National League, which held that baseball is not interstate commerce and therefore not subject to federal antitrust law; this ruling has been weakened only in subsequent years. The weakened ruling granted more stability to the owners of teams and has resulted in values increasing at double-digit rates. There were several challenges to MLB's primacy in the sport between the 1870s and the Federal League in 1916.
The chief executive of MLB is the commissioner Rob Manfred. The chief operating officer is Tony Petitti. There are five other executives: president, chief communications officer, chief legal officer, chief financial officer, chief baseball officer; the multimedia branch of MLB, based in Manhattan, is MLB Advanced Media. This branch oversees each of the 30 teams' websites, its charter states that MLB Advanced Media holds editorial independence from the league, but it is under the same ownership group and revenue-sharing plan. MLB Productions is a structured wing of the league, focusing on video and traditional broadcast media. MLB owns 67 percent of MLB Network, with the other 33 percent split between several cable operators and satellite provider DirecTV, it operates out of studios in Secaucus, New Jersey, has editorial independence from the league. In 1920, the weak National Commission, created to manage relationships between the two leagues, was replaced with the much more powerful Commissioner of Baseball, who had the power to make decisions for all of professional baseball unilaterally.
From 1901 to 1960, the American and National Leagues fielded eight teams apiece. In the 1960s, MLB expansion added eight teams, including the first non-U. S. Team. Two teams were added in the 1970s. From 1969 through 1993, each league consisted of an West Division. A third division, the Central Division, was formed in each league in 1994; until 1996, the two leagues met on the field only during the All-Star Game. Regular-season interleague play was introduced in 1997. In March 1995 two new franchises, the Arizona Diamondbacks and Tampa Bay Devil Rays, were awarded by MLB, to begin play in 1998; this addition brought the total number of franchises to 30. In early 1997, MLB decided to assign one new team to each league: Tampa Bay joined the AL and Arizona joined the NL; the original plan was to have an odd number of teams in each league, but in order for every team to be able to play daily, this would have required interleague play to be scheduled throughout the entire season. However, it
Harvard University is a private Ivy League research university in Cambridge, with about 6,700 undergraduate students and about 15,250 postgraduate students. Established in 1636 and named for its first benefactor, clergyman John Harvard, Harvard is the United States' oldest institution of higher learning, its history and wealth have made it one of the world's most prestigious universities; the Harvard Corporation is its first chartered corporation. Although never formally affiliated with any denomination, the early College trained Congregational and Unitarian clergy, its curriculum and student body were secularized during the 18th century, by the 19th century, Harvard had emerged as the central cultural establishment among Boston elites. Following the American Civil War, President Charles W. Eliot's long tenure transformed the college and affiliated professional schools into a modern research university. A. Lawrence Lowell, who followed Eliot, further reformed the undergraduate curriculum and undertook aggressive expansion of Harvard's land holdings and physical plant.
James Bryant Conant led the university through the Great Depression and World War II and began to reform the curriculum and liberalize admissions after the war. The undergraduate college became coeducational after its 1977 merger with Radcliffe College; the university is organized into eleven separate academic units—ten faculties and the Radcliffe Institute for Advanced Study—with campuses throughout the Boston metropolitan area: its 209-acre main campus is centered on Harvard Yard in Cambridge 3 miles northwest of Boston. Harvard's endowment is worth $39.2 billion, making it the largest of any academic institution. Harvard is a large residential research university; the nominal cost of attendance is high, but the university's large endowment allows it to offer generous financial aid packages. The Harvard Library is the world's largest academic and private library system, comprising 79 individual libraries holding over 18 million items; the University is cited as one of the world's top tertiary institutions by various organizations.
Harvard's alumni include eight U. S. presidents, more than thirty foreign heads of state, 62 living billionaires, 359 Rhodes Scholars, 242 Marshall Scholars. As of October 2018, 158 Nobel laureates, 18 Fields Medalists, 14 Turing Award winners have been affiliated as students, faculty, or researchers. In addition, Harvard students and alumni have won 10 Academy Awards, 48 Pulitzer Prizes and 108 Olympic medals, have founded a large number of companies worldwide. Harvard was established in 1636 by vote of the Great and General Court of the Massachusetts Bay Colony. In 1638, it acquired British North America's first known printing press. In 1639, it was named Harvard College after deceased clergyman John Harvard, an alumnus of the University of Cambridge, who had left the school £779 and his scholar's library of some 400 volumes; the charter creating the Harvard Corporation was granted in 1650. A 1643 publication gave the school's purpose as "to advance learning and perpetuate it to posterity, dreading to leave an illiterate ministry to the churches when our present ministers shall lie in the dust".
It offered a classic curriculum on the English university model—many leaders in the colony had attended the University of Cambridge—but conformed to the tenets of Puritanism. It was never affiliated with any particular denomination, but many of its earliest graduates went on to become clergymen in Congregational and Unitarian churches; the leading Boston divine Increase Mather served as president from 1685 to 1701. In 1708, John Leverett became the first president, not a clergyman, marking a turning of the college from Puritanism and toward intellectual independence. Throughout the 18th century, Enlightenment ideas of the power of reason and free will became widespread among Congregational ministers, putting those ministers and their congregations in tension with more traditionalist, Calvinist parties; when the Hollis Professor of Divinity David Tappan died in 1803 and the president of Harvard Joseph Willard died a year in 1804, a struggle broke out over their replacements. Henry Ware was elected to the chair in 1805, the liberal Samuel Webber was appointed to the presidency of Harvard two years which signaled the changing of the tide from the dominance of traditional ideas at Harvard to the dominance of liberal, Arminian ideas.
In 1846, the natural history lectures of Louis Agassiz were acclaimed both in New York and on the campus at Harvard College. Agassiz's approach was distinctly idealist and posited Americans' "participation in the Divine Nature" and the possibility of understanding "intellectual existences". Agassiz's perspective on science combined observation with intuition and the assumption that a person can grasp the "divine plan" in all phenomena; when it came to explaining life-forms, Agassiz resorted to matters of shape based on a presumed archetype for his evidence. This dual view of knowledge was in concert with the teachings of Common Sense Realism derived from Scottish philosophers Thomas Reid and Dugald Stewart, whose works were part of the Harvard curriculum at the time; the popularity of Agassiz's efforts to "soar with Plato" also derived from other writings to which Harvard students
In physics and fluid mechanics, a boundary layer is an important concept and refers to the layer of fluid in the immediate vicinity of a bounding surface where the effects of viscosity are significant. In the Earth's atmosphere, the atmospheric boundary layer is the air layer near the ground affected by diurnal heat, moisture or momentum transfer to or from the surface. On an aircraft wing the boundary layer is the part of the flow close to the wing, where viscous forces distort the surrounding non-viscous flow. Laminar boundary layers can be loosely classified according to their structure and the circumstances under which they are created; the thin shear layer which develops on an oscillating body is an example of a Stokes boundary layer, while the Blasius boundary layer refers to the well-known similarity solution near an attached flat plate held in an oncoming unidirectional flow and Falkner–Skan boundary layer, a generalization of Blasius profile. When a fluid rotates and viscous forces are balanced by the Coriolis effect, an Ekman layer forms.
In the theory of heat transfer, a thermal boundary layer occurs. A surface can have multiple types of boundary layer simultaneously; the viscous nature of airflow reduces the local velocities on a surface and is responsible for skin friction. The layer of air over the wing's surface, slowed down or stopped by viscosity, is the boundary layer. There are two different types of boundary layer flow: turbulent. Laminar boundary layer flow The laminar boundary is a smooth flow, while the turbulent boundary layer contains swirls or "eddies." The laminar flow is less stable. Boundary layer flow over a wing surface begins as a smooth laminar flow; as the flow continues back from the leading edge, the laminar boundary layer increases in thickness. Turbulent boundary layer flow At some distance back from the leading edge, the smooth laminar flow breaks down and transitions to a turbulent flow. From a drag standpoint, it is advisable to have the transition from laminar to turbulent flow as far aft on the wing as possible, or have a large amount of the wing surface within the laminar portion of the boundary layer.
The low energy laminar flow, tends to break down more than the turbulent layer. The aerodynamic boundary layer was first defined by Ludwig Prandtl in a paper presented on August 12, 1904 at the third International Congress of Mathematicians in Heidelberg, Germany, it simplifies the equations of fluid flow by dividing the flow field into two areas: one inside the boundary layer, dominated by viscosity and creating the majority of drag experienced by the boundary body. This allows a closed-form solution for the flow in both areas, a significant simplification of the full Navier–Stokes equations; the majority of the heat transfer to and from a body takes place within the boundary layer, again allowing the equations to be simplified in the flow field outside the boundary layer. The pressure distribution throughout the boundary layer in the direction normal to the surface remains constant throughout the boundary layer, is the same as on the surface itself; the thickness of the velocity boundary layer is defined as the distance from the solid body to the point at which the viscous flow velocity is 99% of the freestream velocity.
Displacement thickness is an alternative definition stating that the boundary layer represents a deficit in mass flow compared to inviscid flow with slip at the wall. It is the distance by which the wall would have to be displaced in the inviscid case to give the same total mass flow as the viscous case; the no-slip condition requires the flow velocity at the surface of a solid object be zero and the fluid temperature be equal to the temperature of the surface. The flow velocity will increase within the boundary layer, governed by the boundary layer equations, below; the thermal boundary layer thickness is the distance from the body at which the temperature is 99% of the freestream temperature. The ratio of the two thicknesses is governed by the Prandtl number. If the Prandtl number is 1, the two boundary layers are the same thickness. If the Prandtl number is greater than 1, the thermal boundary layer is thinner than the velocity boundary layer. If the Prandtl number is less than 1, the case for air at standard conditions, the thermal boundary layer is thicker than the velocity boundary layer.
In high-performance designs, such as gliders and commercial aircraft, much attention is paid to controlling the behavior of the boundary layer to minimize drag. Two effects have to be considered. First, the boundary layer adds to the effective thickness of the body, through the displacement thickness, hence increasing the pressure drag. Secondly, the shear forces at the surface of the wing create skin friction drag. At high Reynolds numbers, typical of full-sized aircraft, it is desirable to have a laminar boundary layer; this results in a lower skin friction due to the characteristic velocity profile of laminar flow. However, the boundary layer thickens and becomes less stable as the flow develops along the body, becomes turbulent, the process known as boundary layer transition. One way of dealing with this problem is to suck the boundary layer away through a porous surface; this can reduce drag, but is impractical due to its mechanical complexity and the power required to move the air and dispose of it.
Natural laminar flow techniques push the boundary layer transition aft by reshaping the airfoil or fuselage so
A four-seam fastball called a rising fastball, a four-seamer, or a cross-seam fastball, is a pitch in baseball. It is a member of the fastball family of pitches and is the hardest ball thrown by a pitcher; the name of the pitch derives from the fact that with every rotation of the ball as it is thrown, four seams come into view. A few pitchers at the major league level can sometimes reach a pitch speed of up to 100 mph, it is compared with the two-seam fastball. The four-seam fastball is designed purely for velocity; the ball is gripped with the index and middle fingers set on or across a line of the "horseshoe" seam that faces outward, i.e. away from the pitcher's body. The thumb is placed directly underneath the ball; the four-seam fastball is thrown with a straight overhead swing of the throwing arm. The ball leaves the thumb at the top of the throwing motion as the index and middle fingers play their grip on the "top" seam to roll it down the "back" of the ball, which imparts backspin to the ball that lasts the distance of the pitch.
The backspin affects the exchange of momentum between ball and surrounding air such that a lifting force called the Magnus effect offsets the downward pull of gravity on the ball. Further, backspin combined with the steady rotation of four seams in alignment with the direction of the pitch stabilizes the ball's flight-path. A successful four-seam fastball overpowers the batter with velocity zipping through the strike zone before the batter can timely commit to swing; the faster a four-seamer pitch is thrown, the more effective it will be. It is difficult for a batter to get "around on" the pitch—to swing the bat around to meet the ball—because he/she must swing early to "catch up" to the speedy pitch. One of the most dramatic and frequent tableaus in baseball is that of a frustrated batter helplessly swinging "empty" on a fastball that has passed the hitting zone, has made the catcher's mitt. Conversely, because the four-seamer doesn't break, it is quite hittable by the quick, "good-eye" batter who can "see" where the pitch will arrive.
Moreover, its extreme velocity helps experienced batters to hit it hard. Further, a fastball's effectiveness decreases if it is not thrown, i.e. if the pitch is not under control. Due to its straight and level flight an errant fastball will not fool many batters as to its direction; as a pitcher's fastball loses "heat", more batters will have sufficient time to read and hit the pitch. Pitching or throwing a fastball "comes naturally" to most athletes who throw baseballs; the fastball is one of the first pitches taught to young pitchers. It requires little unnatural motion of the arm, elbow or shoulders, the ball comes off the fingers when the pitch is completed as it is intended to be thrown; the fastball is the most common of pitches, as all pitchers throw a fastball as part of their standard repertoire. Scientific studies have shown that the four-seam and two-seam fastballs have the same flight paths and speeds, but a batter perceives a difference between them; the perceived difference is due to flicker fusion threshold, defined as the frequency that a flashing light appears "steady" to the human eye.
For example, for a series of flashed still-pictures to appear steady, the frequency of flashing has to be at a rate greater than the flicker fusion threshold, which for humans is about 60 Hz, or 60 cycles per second. A major league pitcher throws a baseball with a spin of around 20 rotations per second. With each rotation, a four-seam fastball presents four seams crossing the vision of the batter, producing a flicker rate of 80 Hz, which results in the batter not perceiving any features on the ball and having fewer visual cues than with the two-seamer to track it. Thus, the batter perceives the four-seam fastball as faster and higher than a two-seam fastball
Jay Hanna "Dizzy" Dean known as Jerome Herman Dean, was an American professional baseball pitcher. During Dean’s Major League Baseball career, he played for the St. Louis Cardinals, Chicago Cubs, St. Louis Browns. A brash and colorful personality, he was the last National League pitcher to win 30 games in one season. After his playing career, “Ol’ Diz” became a popular television sports commentator. Dean was elected to the Baseball Hall of Fame in 1953; when the Cardinals reopened the team Hall of Fame in 2014, Dean was inducted among the inaugural class. Born on January 16, 1910 in Lucas, Ark. Dean attended public school only through second grade, his colorful personality and eccentric behavior earned him the nickname "Dizzy". He made his professional debut in 1930 and worked his way up to the major leagues that same year, throwing a complete game three-hitter for the Cardinals. Dean was best known for winning 30 games in the 1934 season while leading the 1934 "Gashouse Gang" St. Louis team to the National League pennant and the World Series win over the Detroit Tigers.
He had a 30–7 record with a 2.66 ERA during the regular season. His brother, was on the team, with a record of 19-11, was nicknamed "Daffy", although this was only done for press consumption. Though "Diz" sometimes called his brother "Daf", he referred to himself and his brother as "Me an' Paul". Continuing the theme, the team included Joe "Ducky" Medwick; the Gashouse Gang was the southernmost and westernmost team in the major leagues at the time, became a de facto "America's Team." Team members Southerners such as the Dean brothers and Pepper Martin, became folk heroes in the Depression-ravaged United States. Americans saw in these players and hustling rather than handsome and graceful, a spirit of hard work and perseverance, as opposed to the haughty paid New York Giants, whom the Cardinals chased for the National League pennant. Much like sports legends Joe Namath and Muhammad Ali, Dizzy liked to brag about his prowess and make public predictions. In 1934, Dizzy predicted, "Me an' Paul are gonna win 45 games."
On September 21, Diz pitched no-hit ball for eight innings against the Brooklyn Dodgers, finishing with a three-hit shutout in the first game of a doubleheader, his 27th win of the season. Paul threw a no-hitter in the nightcap, to win his 18th, matching the 45 that Diz had predicted. "Gee, Paul", Diz was heard to say in the locker room afterward, "if I'd a-known you was gonna throw a no-hitter, I'd a-throw'ed one too!" On May 5, 1937, he bet. He struck him out his first three at-bats, but when DiMaggio hit a popup behind the plate at his fourth, Dean screamed at his catcher, "Drop it!, Drop it!" The catcher did and Dean fanned DiMaggio, winning the bet. Few in the press now doubted Diz's boast, as he was fond of saying, "If ya done it, it ain't braggin'." Diz finished with 30 wins, the only NL pitcher to do so in the post-1920 live-ball era, Paul finished with 19, for a total of 49. The Cards needed them all to edge the Giants for the pennant, setting up a matchup with the American League champion Detroit Tigers.
After the season, Dizzy Dean was awarded the National League's Most Valuable Player Award. Dean was known for antics. In time, perception became reality. In Game 4 of the 1934 World Series against the Detroit Tigers, Dean was sent to first base as a pinch runner; the next batter hit a potential double play groundball. Intent on avoiding the double play, Dean threw himself in front of the throw to first; the ball struck him on the head, Dean was knocked unconscious and taken to a hospital. The storied sports-section headline the next day said, "X-ray of Dean's head reveals nothing." The St. Louis Post-Dispatch and the Detroit Free Press stated that the X-rays "revealed no lasting injury". However, Dean was reported saying. Although the Tigers went on to win the game 10-4, Dean recovered in time to pitch in Game 5 which he lost. After the Cardinals won Game 6, Dean came back and pitched a complete game shutout in Game 7 to win the game and the Series for the Cardinals. In the World Series the Dean brothers accounted with two each, of the Cardinals wins.
While pitching for the NL in the 1937 All-Star Game, Dean faced Earl Averill of the American League Cleveland Indians. Averill hit a line drive back at the mound. Told that his big toe was fractured, Dean responded, "Fractured, the damn thing's broken!" Coming back too soon from the injury, Dean changed his pitching motion to avoid landing as hard on his sore toe enough to affect his mechanics. As a result, he hurt his arm. By 1938, Dean's arm was gone. Nonetheless, Chicago Cubs scout Clarence "Pants" Rowland was given the unenviable job of obeying owner P. K. Wrigley's order to buy the washed-up Dizzy Dean's contract at any cost. Rowland signed the ragged righty for $185,000, one of the most expensive loss-leader contracts in baseball history. Dean helped; the Cubs had been in third place, six games behind the first place Pittsburgh Pirates led by Pie Traynor. By September 27, with one week left in the season, the Cubs had battled back to within a game and a half game of the Pirates in the National League standings as the two teams met for a crucial three-game series.
Dean pitched the opening game of the series and with an ailing arm, relied more on his experience and grit to defeat the Pirates by a score of 2 to 1. Dean would call it the greatest outing of his career; the victory cut the Pirates'
An optical illusion is an illusion caused by the visual system and characterized by a visual percept that appears to differ from reality. Illusions come in a wide variety. According to that, there are three main classes: physical and cognitive illusions, in each class there are four kinds: Ambiguities, distortions and fictions. A classical example for a physical distortion would be the apparent bending of a stick half immerged in water. An example for a physiological fiction is an afterimage. Three typical cognitive distortions are the Ponzo, Müller-Lyer illusion. Physical illusions are caused by e.g. by the optical properties of water. Physiological illusions arise in the eye or the visual pathway, e.g. from the effects of excessive stimulation of a specific receptor type. Cognitive visual illusions are the result of unconscious inferences and are those most known. Pathological visual illusions arise from pathological changes in the physiological visual perception mechanisms causing the aforementioned types of illusions.
A familiar phenomenon an example for a physical visual illusion are when mountains appear to be much nearer in clear weather with low humidity than they are. This is; the classical example of a physical illusion is when a stick, half immersed in water appears bent. This phenomenon has been discussed by Ptolemy and was a prototypical example for an illusion. Physiological illusions, such as the afterimages following bright lights, or adapting stimuli of excessively longer alternating patterns, are presumed to be the effects on the eyes or brain of excessive stimulation or interaction with contextual or competing stimuli of a specific type—brightness, position, size, etc; the theory is that a stimulus follows its individual dedicated neural path in the early stages of visual processing and that intense or repetitive activity in that or interaction with active adjoining channels causes a physiological imbalance that alters perception. The Hermann grid illusion and Mach bands are two illusions that are best explained using a biological approach.
Lateral inhibition, where in the receptive field of the retina light and dark receptors compete with one another to become active, has been used to explain why we see bands of increased brightness at the edge of a color difference when viewing Mach bands. Once a receptor is active, it inhibits adjacent receptors; this inhibition creates contrast. In the Hermann grid illusion the gray spots appear at the intersection because of the inhibitory response which occurs as a result of the increased dark surround. Lateral inhibition has been used to explain the Hermann grid illusion, but this has been disproved. More recent empirical approaches to optical illusions have had some success in explaining optical phenomena with which theories based on lateral inhibition have struggled. Cognitive illusions are assumed to arise by interaction with assumptions about the world, leading to "unconscious inferences", an idea first suggested in the 19th century by the German physicist and physician Hermann Helmholtz.
Cognitive illusions are divided into ambiguous illusions, distorting illusions, paradox illusions, or fiction illusions. Ambiguous illusions are pictures or objects that elicit a perceptual "switch" between the alternative interpretations; the Necker cube is a well-known example. Distorting or geometrical-optical illusions are characterized by distortions of size, position or curvature. A striking example is the Café wall illusion. Other examples are the famous Müller-Lyer illusion and Ponzo illusion. Paradox illusions are generated by objects that are paradoxical or impossible, such as the Penrose triangle or impossible staircase seen, for example, in M. C. Escher's Descending and Waterfall; the triangle is an illusion dependent on a cognitive misunderstanding. Fictions are when a figure is perceived though it is not in the stimulus. To make sense of the world it is necessary to organize incoming sensations into information, meaningful. Gestalt psychologists believe one way this is done is by perceiving individual sensory stimuli as a meaningful whole.
Gestalt organization can be used to explain many illusions including the rabbit–duck illusion where the image as a whole switches back and forth from being a duck being a rabbit and why in the figure–ground illusion the figure and ground are reversible. In addition, Gestalt theory can be used to explain the illusory contours in the Kanizsa's Triangle. A floating white triangle, which does not exist, is seen; the brain has a need to see familiar simple objects and has a tendency to create a "whole" image from individual elements. Gestalt means "form" or "shape" in German. However, another explanation of the Kanizsa's Triangle is based in evolutionary psychology and the fact that in order to survive it was important to see form and edges; the use of perceptual organization to create meaning out of stimuli is the principle behind other well-known illusions including impossible objects. Our brain makes sense of shapes and symbols putting them together like a jigsaw puzzle, formulating that which isn't there to that whi