Illusion is described as a perception that is distorted from reality. Optical illusions are those images which deceive us and present a picture that is misleading. The visual illusions have been used in paintings and architecture (especially Greek temples) from ancient times. However, it was in the 19th century that J.J. Oppel and Johannes Mueller started studying this phenomenon carefully. Optical Illusions History The history of optical illusions can be traced back to the 5th century B.C. when Epicharmus first presented the explanation of this phenomenon. Epicharmus believed that even if our mind knows and understands everything clearly, the sensory organs deceive us and present an optical illusion. Another Greek philosopher, Protagoras from the same period had a different opinion on this subject. According to him, it was the environment that fools us and not the senses. Views presented by Epicharmus and Protagoras added to the confusion about what actually are optical illusions. The famous Greek philosopher, Aristotle tried to provide some satisfactory answer on this topic. He agreed with Protagoras on the point that we can rely on the senses to obtain a correct picture of reality; however, Aristotle (350 B.C.) also added that it was possible to fool the senses quite easily. In the course of time knowledge about optical illusions was enriched by different philosophers and researchers. Thoughts presented by Plato also provide some insight into this subject matter. According to Plato, deciphering the trickery and ultimately the reality behind illusions is possible with the use of both senses and mind. One of the examples of optical illusions from the past is associated with roof-tops of Greek temples. Roofs of these temples were built in a slanting manner. This method of construction created an illusion that roofs were horizontal. This happened because the roof and the walls, when exactly perpendicular to each other, gave the illusion that the roof was curved or bowed. A scientific explanation of optical illusions should help you gain an in-depth understanding of this concept. Mueller and Oppel In the 19th century, Johannes Mueller and J.J. Oppel conducted exhaustive study on the subject of visual illusions. They wrote numerous articles and books on these illusions. People developed interest for this topic once again due to the study conducted by these psychologists. The Mueller-Lyer illusion helped in understanding the concept of illusion in a better manner. As many as 12 theories were proposed to explain this example of illusion. Cognitive Illusions Hermann von Helmholtz (19th century), a physicist from Germany put forth the idea of cognitive illusion. According to Helmholtz, cognitive illusions arise from assumptions a person holds about his environment or world as a whole. The human brain and eyes make unconscious inferences based on these assumptions and thereby create cognitive illusions. Distorting illusion is another form of cognitive illusion. Cafe Wall Illusion is the best example of this illusion. The size, curvature, length, etc., of an image seems to be distorted in distorting illusion. Another type of cognitive illusion is the fictional illusion; non-existent objects seen by people suffering from schizophrenia and other such disorders are referred to as fictional illusions. Cartoons Made By W.E. Hill The famous cartoon made by W.E. Hill in 1915 is a perfect example of optical illusion. In this cartoon, images of an old and young woman were merged together. It is our perception that leads to what we see in the picture. Watching the sketch carefully reveals that different images are merged together skillfully. The Dawn of Op Art In the 60s decade, there was growing interest among abstract painters in 'Op Art'. Paintings classified under Op Art present impression of hidden images, vibrations, flashing and other patterns. Artists like Bridget Riley and Vasarely greatly popularized this style of painting. Other notable artists in this league were Kenneth Noland, Jesus-Rafael Soto, Richard Anuszkiewicz, etc. The history of optical illusions presented in the article profiles the works of notable philosophers, painters and researchers who studied this phenomenon. It is interesting to study how the phenomenon of illusion has an effect on our perceptions and day-to-day life. I hope this article helped you to understand the history and phenomenon of optical illusion well.