Put another way, optical illusions occur when our eyes send information to our brains that tricks us into perceiving something that does not match reality.
One of the more famous optical illusions is the drawing of a “rabbit-duck” which appeared in Harper’s Weekly in 1892. When looking at the ambiguous drawing the brain switches between seeing a rabbit and a duck. The American psychologist Joseph Jastrow used the figure to point out that perception is not just a product of the visual stimulus, but also of mental activity. Interestingly, children tested on Easter Sunday are more likely to see the figure as a rabbit, whereas when tested on a Sunday in October, they tend to see it as a duck.
The Cathedral of the Assumption of the Blessed Virgin Mary in Gozo, Malta does not have a domed roof, it’s an optical illusion.
Still images that appear to move
Artists can create optical illusions that trick us into thinking static images are moving, but scientists aren’t entirely sure why our brains perceive such motion. What you’re experiencing is illusory motion, an optical illusion in which a static image appears to move. The effect is the result of interacting colour contrasts and shape position.
Magic Eye Picture
Neuroscientist and psychologist Bela Julesz invented the stereogram in 1959 to test people’s ability to see in 3D and support his idea that depth perception happened in the brain and not in the eye. Magic Eye images look like a random collection of dots until you look at them in just the right way, and they become a 3D object.
Hold the centre of the printed image right up to your nose. Focus as though you are looking through the image into the distance. Very slowly move the image away from your face until the hidden image appears like magic. Once you see the image, you can look around the entire 3D image. The longer you look, the clearer the illusion becomes. (Answer at the bottom of the page.)
A mirage is a naturally occurring optical phenomenon in which light rays bend to produce a displaced image of distant objects or the sky. It is an illusion sometimes seen at sea or in the desert.
3D Floor pattern in multi-coloured marble
The floor of the Cathedral of Santa Maria del Fiore in Florence viewed from above appears to have depth. (See featured image collage.) Construction on the cathedral was begun in 1296 and completed in 1436.
3D Sidewalk Art
If you want to understand the science behind 3D pavement art, it’s best to look at the image with only one eye. If you use both eyes your brain will tell where the light is coming from, spoiling the illusion.
Answer Magic Eye picture: Shark