Q.Why do illusions occur?

Illusions occur because our perceptual system does not passively record the world exactly as it is but actively interprets sensory information, and on certain occasions this interpretation goes wrong even though the underlying stimulus is perfectly normal. …

Illusions occur essentially because perception is not a passive, mirror-like recording of the physical world exactly as it is, but an active process of interpretation -- and on certain occasions, that interpretation goes wrong, producing a genuine mismatch between the actual physical stimulus present and what we go on to perceive.

Such systematic misperceptions, resulting specifically from misinterpreting the information received by our sense organs, are what psychologists term illusions. They are distinguished from other kinds of perceptual departures from reality by two important features: an illusion is always triggered by a genuinely present external stimulus (unlike a hallucination, which occurs without any corresponding external stimulus at all), and a true illusion tends to be experienced in broadly the same way by most people exposed to it -- which is why illusions are sometimes described as 'primitive organisations' of the perceptual system, meaning they arise in a fairly automatic, built-in fashion rather than through deliberate reasoning that then happens to go wrong.

Illusions can, in principle, be produced through the stimulation of any of our senses, but visual illusions have been studied and documented far more thoroughly by psychologists than illusions in any other sense modality. Different specific illusions can arise from different underlying causes -- some, like the geometrical illusions (Muller-Lyer, vertical-horizontal), seem to be built into the way particular stimulus arrangements are automatically processed by the visual system, sometimes influenced by the perceiver's habitual, learned experience with similar shapes and angles in their own physical environment (this is part of why susceptibility to specific illusions can differ across cultural groups, as with the Segall, Campbell and Herskovits cross-cultural findings); others, like the apparent-movement (phi-phenomenon) illusion, arise from how the visual system processes a rapid sequence of separate, motionless stimuli as though they represented continuous motion. …