Optical illusions that fool your brain

Both lines are exactly the same length.

This is the Müller-Lyer illusion, first described in 1889. The fins pointing outward make a line look longer, and fins pointing inward make it look shorter. One popular explanation is that our brains read the fins as corners of rooms and buildings and adjust the size we perceive.

The two orange circles are identical.

Known as the Ebbinghaus illusion, it shows that we judge size by comparing with surroundings. Ringed by big circles, the center looks small; ringed by small circles, it looks large.

The bars are the same length.

The Ponzo illusion uses converging lines, like railway tracks, to suggest distance. The brain assumes the upper bar is farther away, so it scales it up and we see it as longer.

There is no triangle drawn at all.

The Kanizsa triangle shows how the brain fills in missing information. The cut-out circles suggest edges, and we perceive a bright triangle that is not on the page.

Every gray line is perfectly straight and parallel.

Named after a café wall where it was noticed in the 1970s, this illusion comes from the offset black and white tiles. The contrast at each edge tricks the visual system into seeing wedges.

Both squares are exactly the same gray.

This is simultaneous contrast. The same gray looks lighter on a dark background and darker on a light one, because the eye judges brightness relative to its surroundings.

The long vertical lines are perfectly parallel.

In the Zöllner illusion, short slanted marks make long parallel lines appear to tilt toward or away from each other.

They are the same size.

The Jastrow illusion: when two identical arcs are stacked, the lower one looks bigger because we compare its long outer edge with the short inner edge of the shape above.

They are the same length.

Most people see the vertical line as longer. This vertical-horizontal illusion is partly explained by the horizontal line being cut in half by the vertical one.

There are no gray dots. The white lines are pure white.

The Hermann grid illusion makes faint gray spots appear at the crossings, which vanish when you look straight at one. It is linked to how cells in the eye respond to light and dark areas next to each other.