Although our senses feel truthful, they do not necessarily accurately reproduce the physical reality of the world around us.
Optical illusions, or visual illusions, are defined by “the dissociation between the physical reality and the subjective perception of an object or event.” When we experience an optical illusion, we often see something that is not there or fail to see something that is there.
There are three main types of optical illusion:
Literal optical illusions
These create images that are different from the smaller images or objects that make them. This picture of an elephant is a fairly standard example of a literal optical illusion.
These are the effects of excessive stimulation of a specific type such as brightness or color, such as an ‘afterimage’. A common physiological afterimage is the dim area that seems to float before one's eyes after briefly looking into a bright light source, such as a camera flash.
These arise as a result of unconscious inferences and are what most people think about when they consider optical illusions.
Cognitive illusions perhaps give the greatest insight into how our minds work as they are thought to arise when an image we see interacts with the assumptions we hold about the world, leading to "unconscious inferences". This idea was first suggested in the 19th century by the German physicist and physician Hermann Helmholtz.
Cognitive illusions are usually divided into four categories:
Ambiguous illusions: Pictures or objects that elicit a perceptual 'switch' between the alternative interpretations. The Rubin's Vase illusion is an ambiguous illusion.
Distorting illusions: These are characterized by distortions of size, length, position or curvature.
In the Ponzo Illusion the upper line looks longer because we interpret the converging sides according to linear perspective as parallel lines receding into the distance.
Paradox illusions: Illusions that are generated by objects that are paradoxical or impossible, such as this Penrose Triangle.
Fiction illusions: When an object is perceived even though it is not in the image, such as this Kanizsa Triangle.