Optical Illusions are images that deceive human eyes into believing what is not reality – the image transferred through the eyes are processed on the basis previous experience or known information into visual perceptions that are far removed from reality.
They are broadly classified into literal illusions, physiological illusions, cognitive illusions or unconscious inferences.
Literal illusions are objects that the eyes perceive that are composed of different objects that a close scrutiny would reveal. Example – the face of a celebrity composed of a single typed letter filling an outline that gives the illusion of a face, a picture of flowers composed entirely of appropriately colored and shaped fruits.
Physiological illusions are those that occur as a result of how our eyes function, in the process and timing of capturing and retaining images and moving on to the next. We believe these are seamless activities. But in reality, shifting intensities of light show that we carry lingering afterimages that alter our perceptions. The 3D posters – popular some time ago – that required us to hold them at an angle to the light and focus intently on a point to see the hidden images is an example of this.
Cognitive illusions are those that call on us to consciously submerge a truth known to us and participate actively in believing an illusion created in front of our eyes. Best examples of this type are the illustrations of the Dutch graphic artist M C Escher, the best known of which are the Ascending and Descending and the Waterfall.