The greatest optical illusions of all time

Your brain usually operates by a specific set of defined rules. Optical illusions throw that visual rulebook out a the window. These sneaky images trick your brain into “seeing” things that might not really be there. Here are some of the greatest optical illusions of all time that will really blow your mind.

Troxler’s Effect

A black cross over a colorful out of focus picture
Credit: Bautsch/ CC0

Your brain is lazy. Because there’s so much information entering and exiting your brain every millisecond, it has to come up with ways to conserve energy. Sometimes, if there’s no movement to stimulate your eyes, they simply stop paying attention, and whatever you’re staring at starts to fade away.

In 1804, Swiss physician and philosopher Ignaz Paul Vital Troxler came up with a clever way to force this phenomenon. He made an out-of-focus picture and put a black cross in the middle. If you focus your gaze on the cross long enough, the image will slowly disappear. This optical illusion became known as the Troxler Effect.

Adelson’s Checker Shadow Illusion

If you look at this picture of a green cylinder casting a shadow over a small checkerboard, you might not even notice that it’s an optical illusion. The square labeled “A” is in a dark square outside of the shadow, and the square labeled “B” is in a white square inside the shadow. The square labeled “B” seems a lot lighter than the square labeled “A,” right? They’re actually the exact same shade of gray!

This illusion takes advantage of your brain’s ability to distinguish brightly lit dark surfaces and dimly lit bright surfaces. In life, this ability helps humans distinguish edges, shapes and colors in all lighting. By making both squares the same shade of gray and tricking your brain into thinking one is in a dimmer light, you see them as two different colors.

The Poggendorff Illusion

An illustration of the poggendorff illusion
Credit: Zhitkov Boris/ Shutterstock

On one side of the gray rectangle is a single black line. On the other side is a red line and a blue line. If the gray rectangle went away and the lines were connected, which colored line would connect with the black one?

The answer to this infuriating illusion is that the black line would connect with the red one. No matter how many times you look at it, it’s not going to stop being true. If you want proof, tilt your screen until the colored lines are vertical — or get a ruler. The workings behind this illusion are still unknown, but for some reason, the illusion doesn’t work when it’s rotated.

Shepard’s Tables

Drawing of two tables
Credit: Lbeaumont/ CC BY-SA 4.0

There are two tables. One is facing north to south, and the other east to west. The one facing east to west looks quite obviously bigger, but when you lay them on top of one another, it turns out they’re exactly the same size.

This illusion was discovered by cognitive scientist Roger Shepard. It messes with your brain’s ability to see things in three-dimensions. Although both tables have the exact same surface area, your brain perceives the one aligned north to south to be long and skinny, as if it’s facing away from you.

Kanizsa’s Triangle

There are only three circles with missing pieces and three angles in this optical illusion. The two triangles — the white one with a point facing up and the black outlined triangle with the point down — don’t actually exist. Kanizsa’s Square shows a similar illusion with four circles with a quarter missing from each.

This illusion was created by Italian artist and psychologist Gaetano Kanizsa in 1955. The exact mechanisms in your eyes and brain for this one aren’t entirely understood, but it plays on your brain’s ability to perceive edges that are created by luminance, color and texture.

Impossible Trident

Two illustrations of the impossible trident optical illusion
Credit: Peter Hermes Furian/ Shutterstock

On one side of the picture there are three prongs. On the other side there are only two. Your brain knows that such an object is impossible, but on the other hand, you can't deny what you see.

The impossible trident is probably the most famous of the impossible figures. It was created in 1964 by American psychologist D. H. Schuster. Other psychologists and artists, most famously M.C. Esher, enjoyed using impossible figures in their paintings and experiments to mess with your mind.