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> Optical Illusions Issue: 2008-1 Section: Biology








Our perception imposes on us a certain way of evaluating things. The items look bigger when they are well illuminated. A simple phenomenon of antithesis.

The two disks have exactly the same colour but the right one in a lighter background seems darker. (fig.5 left)

The Grey colour, which covers the black bars on the left, looks lighter than the other despite the fact that they are the same. (fig.5 right).


The painters have long known that the sensation of colours in a painting depends on their interactions and that their brightness is mutually affected.


It appears that the retina transmits to the brain concentrated information, in which the boundaries between dark and bright areas are reflected.

Leonardo Da Vinci had noted that the failing snow appears darker when we look at it up in the sky, but all white when we look at it with the dark window of the house opposite as the background.


In the picture six, the drawing consists of one-coloured rectangular shapes, placed one next to the other.

If we cover with the finger the dividing line of two adjoining rectangular shapes, we see both shades to become the equated.


Geometrical Deceits and Perspective

Some writers have claimed that geometrical deceits manifest themselves in environments of formations with strong indicators of perspective. The indicators of perspective cause illusions.

The skiers in Buffond Lyceum (fig.7), have all the same size but the remotest one seems much larger due to the strong indicators of the picture's perspective.

The size stability is hard to be applied in vertical direction. According to Buffond we have learnt to calculate sizes and the distances on a horizontal direction and not in a vertical one.

Therefore, when we are up high on a storey of a block of flats, the people in the street seem much smaller than they would if they were at the same distance on a horizontal level.


The same happens if we look upwards at something that is high up e.g. at a cross on the top of a bell tower. The sculptor intakes the upper part of the statue bigger in order to counter balance this problem, so that it looks in the right proportions when we look at it from a lower position.


The illusion Muller-Lyer consists the most widely known geometrical deceipt. In a, its classic version is pictured. The two horizontal segments are of the same length but the upper looks smaller than the lower.

The illusion is due to the direction of the shafts. In the b design what matters is the presence of longer or shorter straight sections next to the original ones. In c in the variation of the cross the deceit is less strong, in d we see the Judd variation. The dots which are in the middle of the section seem closer to the center than the ends. (fig.8)

In the picture nine, the inner circle surrounded by bigger ones, looks smaller than the relative to the right, another geometrical deceit.



Perception deals with what is permanent as background noise and ignores it. Therefore, we don't have the sensation of the clothes or glasses we wear. Some adjustments of the perception are very quick and cause consequent phenomena (side-effects), where a vivid sensation is followed immediately after its opposite.


When we stare at an object of a certain colour and then we turn our eyes to a white wall, we see an idol of the object in a complementary colour which is called side effect picture. The most effective means to form a meteikasma is the flash light. In a dark room we shed light on an object with a flash light of an ordinary camera and the side effect picture appears without difficulty.



The illusion of motion is due to the phenomenon of the pictures' lingering on the retina. The meteikasma has an average duration of about me eighth of a second, and it was first studied on a scientific level by Isaac Newton in the lace 17th century.

Cinema is based on this illusion. The continuation or merging of the projected pictures' motion is explained with Linke's natural law, according to which, a series of static pictures of a moving object create the illusion that the object is in motion, when two successive pictures are seemingly the same. The interrupted highlighting found in the older movies, no longer exists nowadays because modern projection equipments also send out light to the parts intervening between the pictures.


Illusions and Art

A painting from the painter Isia Leviant's series Enigma. If you look at it for 10 seconds you will see wreaths? Appearing in which it looks as if some fluid is flowing circularly. Other pictures with illusions from the art (M.C. Escher).



  • Jacques Ninio, The science of illusions, Katopro edition.
  • Domi encyclopedia.
  • Physics, B Junior High school, OEDB edition 2005
  • Researchers-I discover the world, magazine issue 30, 2002
  • Avaton Magazine, Issue 59, 2006.
  • Newspaper NEA weekend 24-3-2006


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