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The color optical illusions featured on this page are taken from Sarcone and Waeber's books shown below


optical illusion

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Colors of the Mind


'Color Illusions' by Gianni A. Sarcone and Marie-Jo Waeber


Color only starts to exist when our perception systems produce the impression of ‘color’: light is perceived on the retina as a stimulus and is processed into a perception of color in our brain. In substance, colors are already illusions in themselves...

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Color is energy… in fact it is an electromagnetic phenomenon, which depends on the way that light is reflected on the objects. Every object absorbs a part of the light which hits it and deflects the rest towards our eyes: this reflected light is interpreted by our brain as a particular color. We shouldn’t, therefore, be surprised to find that the word color comes from the Latin root celare (i.e. 'that which covers, conceals'). Color is then already an illusion per se, a ghost that takes life only in our visual system, when light stimulates the photo-receptors - the antennae that pick up luminous signals – that fill in the background of our eyes. The world surrounding us is sadly in reality monochrome.
But there is also another trick: to the eye color is measured partly on the basis of the frequency of light that hits it but above all in relation to colors nearby. A color is perceived to be brighter, for example, if it is surrounded by a complementary color (two colors are said to be complementary if the sum of their radiation equals or gives white) or lighter if the background color is darker. There is then a mechanism that enhances the contrast of the outline of an object
relatively to its background: it is called lateral inhibition, because each group of photo-receptors tends to inhibit the response of the one next to it. The result is that which appears to be clear appears even to be more so and vice versa. The same mechanism works for colors: when a photo-receptor from one area of the retina becomes stimulated by a color, those next to it become less sensitive to that color. So, for example, the light blue of a small square that you see on a blue background, appears to our eyes clearer than it would do on a yellow background (because yellow contains no blue).

color assimilation 1

An alignment of grey bars allows us to discover the effect of color assimilation: the grey bars in contact with blue acquire a similar tone, the same for the part that touches the red color, giving the impression of color gradation. However, the grey is always grey!


Color afterimage effect

Make sure your cursor is moved away from the image below. Stare at the "+" in the middle of the picture for 15 seconds. Without looking away from the "+", move your cursor over the image.

afterimage effect

arrow What happens?

Complementary Contrasts

Which of the strips below, A or B, corresponds to the green strip positioned on the red and green field? This illusion is based on the effect of simultaneous color contrast. arrow See answer

simultaneous contrast

Visual Weave

sinoultaneous contrastKnown as simultaneous brightness contrast the perceptual phenomenon that appears to make a colored area seem clearer when it is placed on a dark background, as in the example opposite. It is evident that the lenticular forms at the bottom are all equal. Note, though, how they appear to be different when they are placed on different backgrounds, light and dark. The difference is accentuated when we add an interwoven background.

This phenomenon has puzzled scientists and philosophers for two millennia, yet there is still no consensus as to exactly why it happens. Interesting observations and comments on this subject are provided by scientists Edward Adelson (lightness induction) and Alexander Logvinenko (lightness-shadow invariance).


The article featured on this page is adapted from our 'Optical Illusions Chronicles' (Visione, in Italian) in Focus Giochi review, issues #5, #6, and #7. If you enjoy optical illusions, why not to subscribe to Focus Giochi and receive them in your mailbox?

brightness contrast 1

Although they appear to be completely different, the tiles a, b and c, in the picture above, are identical in form color and tone. Again it is caused by the effect of simultaneous brightness contrast.

brightness contrast 2

Which ball on the coffee table corresponds exactly with Ball a) on the floor: Ball b) or ball c)?
arrow See answer

Related links

small square Color Illusory Motion
small square Color Blindness Test
small square Depth Color Effect
small square Vintage Optical Illusions


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