In visual perception, an optical illusion is an illusion caused by the visual system and characterized by a visual percept that arguably appears to differ from reality. Illusions come in a wide variety; their categorization is difficult because the underlying cause is often not clear but a classification proposed by Richard Gregory is useful as an orientation. According to that, there are three main classes: physical, physiological, and cognitive illusions, and in each class there are four kinds: Ambiguities, distortions, paradoxes, 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 paradox is the motion aftereffect. An example for a physiological fiction is an afterimage. Three typical cognitive distortions are the Ponzo, Poggendorff, and Müller-Lyer illusion. Physical illusions are caused by the physical environment, 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 perhaps those most widely known.
Depth perception is the ability to perceive distance to objects in the world using the visual system and visual perception. It is a major factor in perceiving the world in three dimensions. Depth perception happens primarily due to stereopsis and accommodation of the eye.
The Oregon Vortex is a roadside attraction that opened to tourists in 1930, located on Sardine Creek in Gold Hill, Oregon, in the United States. It consists of a number of interesting effects, which are gravity hill optical illusions, but which the attraction's proprietors propose are the result of paranormal properties of the area.
An inclinometer or clinometer is an instrument used for measuring angles of slope, elevation, or depression of an object with respect to gravity's direction. It is also known as a tilt indicator, tilt sensor, tilt meter, slope alert, slope gauge, gradient meter, gradiometer, level gauge, level meter, declinometer, and pitch & roll indicator. Clinometers measure both inclines and declines using three different units of measure: degrees, percentage points, and topos. The astrolabe is an example of an inclinometer that was used for celestial navigation and location of astronomical objects from ancient times to the Renaissance.
The Moon illusion is an optical illusion which causes the Moon to appear larger near the horizon than it does higher up in the sky. It has been known since ancient times and recorded by various cultures. The explanation of this illusion is still debated.
The Mystery Spot is a tourist attraction near Santa Cruz, California, opened in 1939 by George Prather. Visitors experience demonstrations that appear to defy gravity, on the short but steep uphill walk and inside a wooden building on the site. It is a popular tourist attraction, and gained recognition as a roadside "gravity box" or "tilted house". The site is what is known as a gravity hill and was the first of its kind to be built in California.
An Ames room is a distorted room that creates an optical illusion. Likely influenced by the writings of Hermann Helmholtz, it was invented by American scientist Adelbert Ames Jr. in 1946, and constructed in the following year.
The Electric Brae is a gravity hill in South Ayrshire, Scotland, where a freewheeling vehicle will appear to be drawn uphill by some mysterious attraction. The phenomenon is an optical illusion.
Polo is a municipality of the Barahona Province in southwestern Dominican Republic, known for its green and high mountains and for growing fine coffee.
Pandariya is a town and Nagar Palika Parishad and development block in Kabirdham district in the Indian state of Chhattisgarh. This block is famous for the Dewanpatpar magnetic hill, 40 km north of Pandariya, which has an optical illusion of a gravity hill where vehicle seems to defy the gravity and roll up the hill.
Parel Vallei is an affluent suburb of Somerset West in the Western Cape, South Africa. There is a high school situated in the suburb known as Parel Vallei High School.
Magnet Hill is a gravity hill located, near Leh in Leh district of Ladakh, India. The layout of the area and surrounding slopes create the optical illusion of a hill. The hill road is actually a downhill road. Objects and cars on the hill road may appear to roll uphill in defiance of gravity when they are, in fact, rolling downhill. It is 7.5 km southeast of Nimmoo and 26.5 km west of Leh on Srinagar-Leh road.
Nimo or Nyemo is a village and the headquarters of an eponymous community development block in the Leh district of Ladakh, India. It is located in the Leh tehsil, 35 km from Leh.
In human visual perception, the visual angle, denoted θ, subtended by a viewed object sometimes looks larger or smaller than its actual value. One approach to this phenomenon posits a subjective correlate to the visual angle: the perceived visual angle or perceived angular size. An optical illusion where the physical and subjective angles differ is then called a visual angle illusion or angular size illusion.
Kalo Dungar or Black Hill is the highest point in Kutch, Gujarat, India, at 462 m (1,516 ft). It is located 97 km (60 mi) from District headquarters of Bhuj and 25 km (16 mi) from nearest town Khavda.
Kimanis–Keningau Highway is a state highway in Sabah, Malaysia, connecting the town of Kimanis to Keningau. It also acts as an alternative for Federal Route 500 from Kota Kinabalu to Tenom which is nowadays somewhat under-use. The 53-kilometre (33 mi) highway began as a main logging road before being upgraded to a full two-lane highway. The highway was opened to traffic in 2006.
Kōkichi Sugihara is a Japanese mathematician and artist known for his three-dimensional optical illusions that appear to make marbles roll uphill, pull objects to the highest point of a building's roof, and make circular pipes look rectangular. His illusions, which often involve videos of three-dimensional objects shown from carefully chosen perspectives, won first place at the Best Illusion of the Year Contest in 2010, 2013, 2018,and 2020 and second place in 2015 and 2016.
The line of sight, also known as visual axis or sightline, is an imaginary line between a viewer/observer/spectator's eye(s) and a subject of interest, or their relative direction. The subject may be any definable object taken note of or to be taken note of by the observer, at any distance more than least distance of distinct vision. In optics, refraction of a ray due to use of lenses can cause distortion. Shadows, patterns and movement can also influence line of sight interpretation.
Wadi al-Baida, locally known as Wadi al-Jinn, is a valley and an anti-gravity hill in al-Baida Park located approximately 30 kilometers northwest of Medina in the Medina Province of Saudi Arabia. Owing to its optical illusion, vehicles left out of gear in the valley appear to be rolling uphill against the gravity, thus, making the area popular among locals and pilgrims to be purportedly haunted by Jinns, the shape-shifting supernatural creatures featured in both pre-Islamic Arabian folklores and Islamic beliefs. However, the Al Madinah Region Development Authority in 2020 rebutted claims of any spiritual presence in the valley.
