5 mind-bending optical illusions and how they work

We break down the science behind the Pac-Man illusion, the world-famous Duck and Rabbit illustration, and more

Doris Wai |

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They say seeing is believing but can you really trust your eyes? From eye-boggling images to logic-defying illusions that play with the subconscious mind, we’ve gathered five famous optical illusions and broken down the science behind them. You might just be surprised by how our eyes (and brain) lie to us far more than you think.

1. Moving snakes  

This psychedelic twirling image will have you staring all day. The circular snakes seem to be rotating like cogwheels, but as soon as you look at the outermost part of a particular snake, it stops moving.

Some visual scientists think this is due to fixation jitter: eye movements that give the illusion that objects near what you’re focusing on are moving.

2. Titchener circles 

Photo: Shutterstock

Which of the two circles in the centre is smaller? The pink circle on the right may seem bigger at first glance, and that’s because that our perception of size changes according to context.

By placing the pink circles inside different sized objects, our brain are tricked into believing one circle is bigger than the other, when they’re actually the same size.   

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3. Lilac Chaser

Also known as the Pac-Man illusion, this image loop shows twelve lilac dots against a gray background, with a black cross in the middle. One dot then disappears and reappears in a clockwise direction. And once you stare at the cross, a green dot “eats up” the flashing dot until all there’s left is a green dot moving in a circuit.

This happens due to an effect known as an “afterimage”. Once the rods and cones cell in our eyes are adjusted to the blinking lilac dots, afterimages are produced and our eyes begin to process colours at the other end of the spectrum, in this case, green.  

4. Mach band

While some optical illusions dupe our brain into seeing motion, others like the Mach bands illusion deceive us into seeing imaginary shades.

The horizontal bar in the image above may look like it’s in different hues of grey when in fact, this is just a trick of the mind. If you cover everything but the bar itself, you’ll find that it’s in one uniform shade.

This is due to our brain interpreting the two ends of the bar placed against different backgrounds. It then figures out what the bar’s true colour would be, and we end up seeing the graduated effect.  

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5. The Duck and Rabbit Illusion

Probably the mother of all optical illusions, this illusion has been around since 1892. And whether you see a duck, a rabbit, or both depends on our mid-level vision (how our brain finds particular edges in the image, and groups information based on things we know).

In the case of the ambiguous image in the duck and rabbit illusion, the edges are unclear, and the animal we end up seeing depends on which parts of the image we focus on.