By playing on the complex feedback loop between the eyes and the brain, optical illusions may fool us into thinking something different than what's actually there.
When our minds wander to different possible meanings of the same picture, we experience the ambiguous figure illusion, a prevalent kind of visual deception.
Lines of equal length can be tricked into seeming unequal in geometric illusions like the well-known Müller-Lyer illusion.
Illusions of motion, in which still pictures seem to move or vibrate, make up another kind.
Perception is inherently subjective and the brain's receptivity to various visual tricks is evidence of how complicated visual processing is.
To put your eyesight to the test, try your hand at this Optical Illusion.
Only one percent of those with exceptionally sharp vision are said to be able to find the concealed letter Y in under six seconds.