In Or Out?
Illusion #39 : Is the dot inside or outside?
An optical illusion happens because the different parts of the eye process images and colours at different speeds, this sometimes means that false images are sent to the brain. The brain receives this information but actually, the eye only sees some visual information at any given time, and the brain processes this and fills in the gaps to give the impression of continuous movement. This is highlighted in a television which actually shows a different image 25 times a second but the brain doesn't see the changes.