Optical Illusions

Optical illusions fascinate us, challenging our default notion that what we see is real.
They demonstrate that all our perception is illusion, in a sense – incoming sensory information is interpreted, yielding the internal representation of the world.
Therefore, after our eyes have filtered the visual input we
need sound judgement of information in order to create our inner reality: “Your senses then you’ll have to trust, /They’ll let you see what’s true and just, / Should reason keep your mind awake”.8 Eyes are just an instrument, actually brains sees the picture.
What is an optical illusion? “I know it when I see one” could not be farther off the track – as the best illusions are the ones where a discrepancy from reality is not ‘seen’ until one uses other modalities (eg. touch) or instruments (rulers, light metres). And even when we know
that we are subject to an optical illusion, most illusory precepts still persist – a phenomenon called cognitive impenetrability.15 As Gregory9 aptly stated it “it is surprisingly hard to define ‘illusion’ in a satisfactory way”.
According to the Merriam-Webster Online Collegiate Dictionary, an illusion is:
1. Something that deceives or misleads intellectually;
2. Perception of something objectively existing in such a way as to cause misinterpretation of its actual nature.

Some illusions are long known to mankind, eg. the waterfall illusion was mentioned by Aristotle: after staring at a waterfall for a couple of minutes neighbouring objects seem to be shifting upwards. This was followed up by Lucretius, Purkinje and Addams who coined the term
‘Waterfall illusion’. Recent evidence suggests that this motion after effect is not due to ‘fatigue’ but rather due to a gain adjustment, an optimal adaptation to prevailing conditions.…