When we look at an optical illusion, we don’t experience an eye malfunction, but rather an error in the brain’s interpretation of visual information. The question is, how can a simple image trick an organ as sophisticated as our brain?

Images are formed at the back of our eye, on the retina, where they’re analyzed dot by dot and then faithfully transmitted to our brain in coded messages. But when our eyes receive contradictory information, the brain has trouble interpreting it correctly. In such cases, the brain will often amplify contrasts and create colours, perspectives, or movement to make sense of what it sees.

There are several different types of optical illusions. Here are two examples:

In this Müller-Lyer illusion, the two horizontal arrows appear to be of different lengths (Image 1). That’s because the arrows are pointing in different directions.

 

Image 1

As we can see in Image 2, however, the arrows actually span the same distance.

Image 2

 

With the example below, the brain tends to amplify the contrasts or create colours to try to make sense of what it’s seeing. The small square to the left therefore seems lighter than the small square to the right, simply because of the green background—they’re actually the same green.

 

Optical illusions are undeniably fascinating. They deceive the senses, show us impossible realities, and even have us second-guessing basic principles of logic, geometry, and physics.