Actually, there are only two colors: one shade of green, and one shade of red! If you find this hard to believe, look at the close-up picture below of where the two red lines cross.
Optical illusions can usually be divided into four types: ambiguous illusions, distorting/geometrical illusions, paradox illusions, and fiction illusions.
1) An ambiguous illusion is one in which the picture may be viewed as one of two or more different images, depending on your perception. Often you may see both, as your brain switches between the two. For example, the duck-rabbit illusion below may be viewed as a duck, or a rabbit.
Another example is the Ponzo Illusion.
The drawings of the artist M. C. Escher display many paradox illusions beautifully. To view some of his works, we recommend visiting the M.C. Escher Official Website, as well as the M.C. Escher page from Artsy.net.
4) A fictional illusion is when an image is perceived in a picture, even though that image is not really there. A good example is the Kanizsa Triangle, shown below.
First, perceptual organization: our minds are constantly trying to make sense of the world around us, and to do this they must organize incoming information into a meaningful whole. This is called Gestalt organization (ref). Gestalt organization can explain many illusions, including the Duck-Rabbit and the Rubin Vase illusions: our minds perceive the whole, but switch back and forth between the duck and the rabbit, or the vase and the faces. It can also explain the Kanizsa triangle, in which the mind perceives triangles by filling in lines within the empty space.
A second explanation of optical illusion is depth and motion perception. In the Ponzo illusion, the lines at the top of the picture appear further away than they are at the bottom of the picture. If this is true, the yellow line at the top is further away, and thus must be larger than the one at the bottom. However, our minds forget that this picture is not really three-dimensional—it is only two dimensional. As such, the lines are really the same size.
Third, as we discussed previously, the perception of color and brightness may make two objects that are the same color and shade appear to be two different shades, as in the red square example from before.
EXPLORE THE WORLD OF OPTICAL ILLUSIONS:
There are many more illusions, and more explanations of how they fool us. Try looking up a few more illusions, and the explanations of how they work. Here are a few to start with:
1) The Hering Illusion
2) The Shepard Illusion
3) The Ebbinghaus Illusion
4) Pinna’s Illusory Intertwining Effect
5) Illusory motion
References for further research:
1. Optical illusion. Wikipedia. 2014, March 31. Retrieved 3-31-2014. (http://en.wikipedia.org/wiki/Optical_illusion).
2. What is an Optical Illusion? Optics 4 Kids. Retrieved 3-31-2014. (http://www.optics4kids.org/home/content/illusions/)
3. Richard G. (1997) Knowledge in perception and illusion. Phil. Trans. R. Soc. Lond. B 352: 1121-1128.
4. Eagleman, D. M. (2001) Visual Illusions and Neurobiology. Nature Reviews Neuroscience. 2(12): 920-926.
5. Purves, D., Lotto, R.B., Nundy, S. (2002) Why We See What We Do. American Scientist 90(3): 236-242.
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