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An introduction to optical illusions and why our eyes sometimes see things that aren't there.
Have you ever looked at a picture that seemed to spin like a fan, only to realize the paper was perfectly still? Why does your brain tell you it's moving when your eyes know it's not?
Your eyes are like high-tech cameras, but your brain is the storyteller that explains the pictures. Usually, they work together perfectly. However, sometimes your brain tries to be too fast. It takes shortcuts to understand the world quickly. When your brain makes a 'guess' about a picture that turns out to be wrong, we call it an optical illusion. It is a moment where your eyes see the truth, but your brain 'sees' a trick! This happens because our brains are programmed to look for patterns and depth, even when they aren't really there.
Quick Check
What is the main reason an optical illusion happens?
Answer
It happens because the brain takes shortcuts to try and understand visual information quickly.
Lines can be very sneaky. One of the most famous tricks is the Müller-Lyer Illusion. Imagine two lines of the exact same length. If you put arrows on the ends pointing inward, the line looks short. If you put arrows pointing outward, the line looks much longer! Your brain sees the outward arrows and thinks it is looking at the inside corner of a room that is far away. Because your brain thinks it is 'farther,' it decides the line must be bigger.
Follow these steps to see the trick: 1. Draw two horizontal lines that are both exactly cm long. 2. On the first line, draw arrows pointing toward the center: . 3. On the second line, draw arrows pointing away from the center: . 4. Look at them from a distance. The second line will look longer even though both are cm!
Quick Check
In the Müller-Lyer illusion, if two lines are both cm, which one looks longer?
Answer
The line with the arrows pointing outward (away from the center) looks longer.
Your brain doesn't just look at an object; it looks at what is around it. This is called simultaneous contrast. If you place a grey square on a dark black background, your brain thinks, 'Wow, that's bright!' But if you move that same grey square onto a bright white background, your brain thinks, 'That looks dark!' Your brain is constantly comparing things to their surroundings to help you see shapes in the shadows.
Imagine a green cylinder standing on a checkerboard. 1. The cylinder casts a shadow over a 'white' square (Square A). 2. Next to it, in the bright light, is a 'dark' square (Square B). 3. Because of the shadow, Square A and Square B might actually be the exact same shade of grey. 4. Your brain 'corrects' Square A to look white because it knows it is in a shadow!
The 'Penrose Triangle' is a drawing that looks like a solid 3D object. 1. Look at the three corners of the triangle. 2. Try to follow the top edge with your finger all the way around. 3. You will find that the lines connect in a way that is mathematically impossible in real life. 4. Your brain is tricked because it tries to force a 2D drawing to follow the rules of 3D space!
What do we call the 'storyteller' that interprets what we see?
If you put a grey circle on a bright white background, how will it look compared to the same circle on a black background?
Optical illusions happen because our eyes are broken.
Review Tomorrow
In 24 hours, try to explain to a friend why a line with outward arrows looks longer than a line with inward arrows.
Practice Activity
Find a picture of a 'Rubin's Vase' illusion. Can you see both the vase and the two faces? Try to make your brain 'switch' between them as fast as possible!