Q.Explain why the Muller-Lyer illusion continues to 'fool' us even after we have measured the two lines and know intellectually that they are exactly equal in length.

Our perceptions do not always match physical reality with complete accuracy -- that is, they are not always 'veridical.' On certain occasions we fail to correctly interpret the sensory information reaching us, producing a genuine mismatch between the physical stimulus that is actually present and what we go on to perceive. These systematic misperceptions, which result specifically from misinterpreting the information received by our sense organs (rather than from any fault in the sense organs themselves), are what psychologists call illusions. Illusions are worth distinguishing carefully from other kinds of perceptual 'errors': an illusion is triggered by a real, external stimulus situation -- unlike a hallucination, which involves perceiving something in the complete absence of any corresponding external stimulus -- and a genuine illusion tends to produce broadly the same kind of misperception across most people who view it, which is part of why illusions are sometimes described as 'primitive organisations' of the perceptual system, occurring in a fairly automatic, built-in way rather than through any conscious reasoning. Although illusions can, in principle, be experienced through the stimulation of any of our senses, psychologists have historically studied and documented visual illusions far more extensively than illusions in any other sense modality. Illusions occur in the first place because our perceptual system does not passively record the physical world exactly as it is; instead, it actively constructs an interpretation of incoming stimuli, drawing partly on built-in features of how …