You can really see it: those lines are crooked! But if you check it with a ruler or a piece of paper, they appear to run perfectly straight. It is an example of an optical illusion, where our eyes perceive something other than what is actually visible. How is this possible?

Our brain interprets everything we see. It puts into perspective the images our eyes ‘record’. This is a conscious choice of Mother Nature. If this were not the case, we would spend hours understanding our environment. Climbing stairs or driving a car was impossible.

Because our brains are always trying to put everything into perspective, we are sometimes deceived by our eyes. Below you will find five examples of optical illusions that show something different than is actually the case.

Cafe wall illusion

With the Café Wall illusion you can clearly see that the horizontal lines are skewed. Yet all white and all black blocks are exactly the same size and the lines are straight. Check it out with a ruler or a piece of paper.

How is this possible? Box thinking plays a major role in this illusion. Our brains have trouble with the chaotic alternation between the black and white areas. So much effort that they ignore the line pattern and judge it completely wrongly as crooked.

Hermann grid

In Hermann’s Grid, the dots you are looking at are white and the dots around them are black. You can see this when you look at one white dot. But the moment you cast your eyes on other dots, they suddenly turn white and the others around them turn black. Crazy!

This is due to the nerve cells in our brain. When we look at something, a nerve cell is activated in our brain. At that moment, the brain suppresses the neighboring nerve cell. They do this to distinguish two stimuli from each other, for example black letters on a white sheet of paper. When we look at a white dot, our brain automatically says ‘the rest is not white’. As a result, we incorrectly see black dots in the corners of our eyes.

Ebbinghaus illusion

Look at the orange dot. Which of the two is bigger? You probably think it’s the judge. But in reality both dots are the same size!

Our brain likes to see objects in relation to other objects in order to judge how big something is. This makes an object surrounded by large objects further away appear smaller than the same object surrounded by small objects closer. While the object is the same size in both cases.

Müller-Lyer illusion

You can make a similar comparison with lines. Just look at the two lines below. The top one seems shorter, right? Actually, they are the same length.

The Müller-Lyer illusion shows how two lines that do not seem to be of equal length at first sight can actually be. When observing the upper figure, our eyes make a kind of U-turn. Our gaze follows the lines ‘inwards’, as it were. When observing the lower figure, our eye movements are wider, and our gaze follows the lines outwards.

The size of our eye movements plays a role in determining the length of an object. This makes the second figure appear longer, while this is not the case.

Adelson checkerboard

The Adelson Checkerboard is for advanced players. You should see very clearly that plane A is darker than plane B. But if you check this with the computer, it turns out that both planes have exactly the same color.

There are two explanations for this. On the one hand, our brain determines the brightness of a surface by looking at the contrast with the other surfaces. As a result, the checkerboard effect (light-dark-light) plays a greater role in determining the color than the shadow effect. On the other hand, our brain automatically takes the effect of shadow into account. Without being true, they therefore think that plane B is actually lighter in reality.

You see: our eyes and brains regularly fool us.


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