Although we may not realize it, or even think about it, we see optical illusions many times a day. Technology, such as television and computer monitors, are always using optical illusions. The images on screen are moving so fast that our brain cannot comprehend each individual frame. So instead, they are blurred together, making the illusion that whatever on screen is moving. Something like a shadow or maybe looking at an object from a certain angle can trick our brains, creating an optical illusion of sorts.
The Greeks were the first people to use optical illusions on purpose for a cause. When building their massive temples, they would make sure that the roofs were slanted. Because of this, the temples were given the illusion of standing straight. They also molded the columns so that they bulged, which in turned created another illusion, that from a distance they were perfectly proportioned.
Optical Illusions are images or objects that are visually misleading in concept. The information that is gathered by the eye is processed by the brain to add up to the something that doesn't follow what the physical measurement of the main source is.
Physiological illusions are illusions caused by exposure to bright lights or excessively longer alternating patterns. The effects on the eyes or the brain being excessively stimulated by a specific type (something like brightness or tilt, for example) will cause an "after image", which is the optical illusion. It is theorized that in the early stages of processing the image certain stimuli are invoked, and due to the repeating stimulation of one or few channels there is a physiological imbalance that occurs that alters perception.
Cognitive Illusions are illusions that occur by interactions with assumptions, which lead to :unconscious inferences'. There are four main types of cognitive illusions: ambiguous illusions, distorting illusions, paradox illusions, and fiction illusions.
Ambiguous illusions are images or objects that provoke a sort of "switch" between two alternate interpretations. A famous example of this type of illusion is the necker cube. The necker cube can be seen when looking at a cube in an isometric perspective, which means that the parallel edges of the cube are drawn as parallel lines in the picture.
Distorting illusions are those illusions that are of images that are visually distorted due to size, length, or curvature. The famous Mueller-Lyer illusion is an example of distorting illusions, which consists of a straight line with arrows on either ends. One on line, these arrows on either side face outwards, on another, inwards. Due to the arrows distorting the view of the line, the line with the arrows facing outwards seem longer, when in fact both lines are exactly the same length.
Paradox Illusions are objects or images in objects that are impossible or create a paradox. M.C. Escher's Ascending And Descending painting is an example of a paradox illusion, with the impossible staircase at the top of the building going around in the square and seemingly going up and up, until it reaches a corner and meets up with the rest of the staircase and is revealed to be the same height all the way around.
Finally, Fictional Illusions are illusions that don't really fit into the other categories, since they are illusions that can only be seen by one person, usually brought on by schizophrenia. They are more properly defined as being hallucinations.
Another simple type of illusion are typography illusions, when something such as a letter or a number in fact looks like something else, such as another number or letter. This would most likely be categorized as an ambiguous illusion, however.
