An optical illusion (also called a visual illusion) is characterized by visually perceived images that differ from objective reality. The information gathered by the eye is processed in the brain to give a percept that does not tally with a physical measurement of the stimulus source. There are three main types: literal optical illusions that create images that are different from the objects that make them, physiological ones that are the effects on the eyes and brain of excessive stimulation of a specific type (brightness, tilt, color, movement), and cognitive illusions where the eye and brain make unconscious inferences.
The Sex Illusion
“In the Illusion of Sex, two faces are perceived as male and female. However, both faces are actually versions of the same androgynous face. One face was created by increasing the contrast of the androgynous face, while the other face was created by decreasing the contrast. The face with more contrast is perceived as female, while the face with less contrast is perceived as male. The Illusion of Sex demonstrates that contrast is an important cue for perceiving the sex of a face, with greater contrast appearing feminine, and lesser contrast appearing masculine.” in words of Richard Russell, Harvard psychologist.
The Waterfall Effect
Stare at the center of the spiral for about a minute. Then, when you focus your gaze at another surface it will appear to be moving around in spiral motion. The spiralling aftereffect was first described by Joseph Plateau (1801–1883) in 1849. For a more powerful effect, click here.
Look at the dot on the image. After about 20 seconds, black and white photograph of spanish castle will appear, but as your eyes adapt to the inverted image you were seeing first, you will see black and white image change to color. This illusion was created by John Sadowski at whose website you can make your own illusion.
The Silhouette Illusion
Take a look at this silhouette of a spinning figurine. Does she spin clockwise or counter clockwise? Created by Nobuyuki Kayahara in 2003.
Created in 2006 at the MFA Computer Art, School of Visual Arts, New York, by Goo-Shun Wang. It is based on creative principles of M. C. Escher.
The Checker Board Shadow
Believe it or not, the squares A and B are the same shade of gray. Designed by Edward H. Adelson, a professor of vision science at MIT. For a full version and detailed explanation, click here.
Only three words necessary to introduce this illusion: “Parental Guidance Recommended”.
Face in the Beans
Look closely and find a face among the beans!
Dynamic Luminance – Gradient Effect
Sit at a comfortable distance and then move forward toward the center of the figure. You will perceive a notable change in brightness and even form — what may be called a “here comes the sun” effect. By moving back and forth, this apparent change will repeat. Created by Alan Stubbs, University of Maine, USA. See illusions and other visual effects at his website PerceptualStuff.
Rotating Snake Illusion
Otherwise known as “anomalous motion illusion”. It was created by Akiyoshi Kitaoka, professor of psychology at Ritsumeikan University, Kyoto, Japan. For a larger view, click on the image below.
If you would like to see more visual phenomena, probably the most extensive, well organized and documented place on the web is Michael Bach’s website.