How does  an optical illusion  work? 

Gestalt organization 

Psychologists who specialize in Gestalt theory believe that the human brain is capable of processing incoming visual information and arranging it into a coherent and meaningful package. 

By way of illustration, the Kanizsa triangle is an example of this type of illusion. In spite of the fact that there is no triangle in existence, the brain manages to integrate the separate components and perceives a white triangle floating in the middle.

Figure–Ground illusion

It would be too much for our senses to handle to see everything at once when we look at the world in front of us. We instead look at one important thing, which is called "the figure." It turns everything else around it into "the ground.

 A figure-ground effect is a picture where it's not clear what the figure and the ground are, so our minds keep going back and forth trying to figure it out. One of the most well-known of these is the Rubin vase, which lets you see either two black faces or a white vase. 

Depth perception illusion

The brain is used to seeing three-dimensional pictures high in the visual field as farther distant and larger. In the Ponzo illusion, two parallel lines above converging lines on a two-dimensional plane create depth, making the higher yellow line appear larger.

People have liked optical tricks for a long time, even before we knew how they worked. But it's still fun to let yourself be fooled, even though improvements in neuroscience have explained how they work visually.

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