This page will make you forget the famous quote “Seeing is believing” with the following optical illusions, but before doing so let’s learn about what an optical illusion is, it is usually defined as an image perceived as deceptive or misleading in the general common sense, also referred to as eye tricks and visual illusions, it’s one of the phenomenon that makes you just feel how wrong we could be when looking at our universe.

How does the optical illusion work?

The information gathered by the eye is processed by the brain to give, on the face of it, a percept that does not tally with a physical measurement of the stimulus source. A conventional assumption is that there are physiological illusions that occur naturally and cognitive illusions that can be demonstrated by specific visual tricks that say something more basic about how human perceptual systems work.

Physiological - Illusions

Physiological illusions, such as the afterimages following bright lights or adapting stimuli of excessively longer alternating patterns (contingent perceptual aftereffect), are presumed to be the effects on the eyes or brain of excessive stimulation of a specific type - brightness, tilt, color, movement, and so on. The theory is that stimuli have individual dedicated neural paths in the early stages of visual processing, and that repetitive stimulation of only one or a few channels causes a physiological imbalance that alters perception.

Cognitive illusions

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 Hermann Helmholtz. Cognitive illusions are commonly 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; another instance is the Rubin vase.

Distorting illusions are characterized by distortions of size, length, or curvature. A striking example is the Café wall illusion. Another example is the famous Mueller-Lyer illusion.

Paradox illusions are generated by objects that are paradoxical or impossible, such as the Penrose triangle or impossible staircases seen, for example, in M. C. Escher's Ascending and Descending and Waterfall. The triangle is an illusion dependent on a cognitive misunderstanding that adjacent edges must join.

Fictional illusions are defined as the perception of objects that are genuinely not there to all but a single observer, such as those induced by schizophrenia or a hallucinogenic substance. These are more properly called hallucinations.

These are some expressions used in different languages to refer to the eye illusion we are looking at above: l'illusion d'optique, l'illusion visuelle, optische illusionen, ilusiones opticas.

Well-known illusions: Ames room illusion , Ames Trapezoid Window illusion, Barberpole illusion, Benham's top, Bezold Effect, Blivet  or the Impossible trident illusion, Cafe wall illusion, Chubb illusion, Cornsweet illusion, Ebbinghaus illusion, Ehrenstein illusion, Flash lag illusion, Flip Book illusion, Fraser spiral illusion, Grid illusion, Hering grid, Hering illusion, Hermann grid illusion, Hollow-Face illusion, Impossible cube, Isometric illusion, Jastrow illusion, Jesus illusion, Kanizsa triangle, Lilac chaser, Mach bands, Missing square puzzle, Moon illusion, Motion aftereffect, Muller-Lyer distortion illusion, Necker cube illusion, Orbison illusion, Penrose triangle aka Impossible triangle illusion, Peripheral drift illusion, Phi phenomenon, Poggendorff illusion, Ponzo illusion, Pulfrich effect or Pulfrich pendulum illusion, Rubin vase, Same color, Sander illusion, Simultaneous brightness contrast, Size-weight illusion, White's illusion, Wundt illusion, Zollner illusion.