Our senses: optical illusions

Do it yourself

Is reality really what we see? Not really. Because our brain always shows us an edited version of what is received through our eyes. Nothing is as it seems! Join us on a journey into the world of optical illusions and learn how and why our eyes and brains trick us a little every day.



Here you will find a variety of experiments that you can use to confuse your eyes, test your friends and solve little puzzles yourself.

Let's go - have fun!

What else can our eyes do?

Would you like to learn more about the structure and function of the eye? Then take a look at Part 1 of our vision series: Of all the Senses: How does vision work?.

Yout want to learn more about the exciting things and tricks our eye can do?
Then you can satisfy your curiosity below in the additional information.
There you will also find more optical illusions and self-tests!


  • Suitable for age group: ~ 9 years old

  • Especially interesting for: Children and teenagers, fans of optical illusions

  • Preparation time: between 10-20 minutes

Experiment 1: Famous optical illusions

In most cases, you can trust your eyes and be confident that the image you see corresponds to reality. However, sometimes the brain makes small mistakes in processing the information and small inaccuracies occur. These deceptions often occur because the brain incorporates the expectations and experiences you have accumulated throughout life. In the following pictures you can experience a few illusions.

Illusion 1: Shadows where there are none

In this picture you see shadows at the points where the white lines cross, although there are none. However, if you focus on only one point of intersection, the shadow disappears there.

Illusion 2: Straight or curved lines?

Look at the thick lines in the two pictures. They appear to be curved outwards or inwards. However, this is not true, because in fact all the thick lines are completely straight. The brain associates the lines with the pattern in the background and is fooled by it.

Illusion 3: Which line is longer?

In these illustrations, you will think at first that the lines are of different lengths. Similar to the example before, the brain is also deceived here by the surrounding elements. In reality, all the lines are the same length.

Illusion 4: Parallel lines?

This figure consists of parallel lines and rectangles of the same size.
Yet it appears to you that the lines are crooked and that the rectangles are trapezoids.

Experiment 2: Optical illusion puzzle

With this simple experiment you can test your perception and puzzle with friends who can estimate best.


  • You need:

  • Two cocktail or martini glasses

  • Tap water

  • A test person/participant

  • Optional: food colouring


Step 1: Preparing

Fill one of your martini glasses to the brim with water. If you like, you can colour the water with a little food colouring to make it easier to see the water level.

Now divide the contents of the full glass evenly between the two glasses.

Step 2: Puzzle over it

Now you are ready and can have your friends guess. Ask your friends, siblings or parents for their opinion. Do they believe that you could pour both glasses together without the contents overflowing?


The conical shape of the jars makes it difficult for you to estimate the volume. You will mainly notice the small difference in height between the almost full and the completely filled jar. You will also have problems estimating the size of the free space because the opening of the jars gets wider towards the top.

Have fun!

Learn more!

Fascinating facts about vision in general

Pupil width adjustment

The width of the pupil is adjusted to the light conditions. However, there are other reasons for a change in pupil width.
For example, if you are excited, you will have larger pupils because your body is trying to sharpen its perception.

Stability of the sclera

The sclera surrounds almost our entire eye and offers it optimal protection. The skin is very similar to a muscle tendon and is so stable that not even a nail could penetrate it.

Why you can't see colours in the twilight

There are two different types of nerve cells on the retina, rods and cones. Rods react mainly in dim light and can distinguish black, white and shades of grey. The cones enable colour vision, but can only work with sufficient, bright light. This is why you can only recognise shadows at dusk, but cannot see any colours.

Myth of the blue eyed babies

Even though 90% of all people have brown eyes, most light-skinned babies are born with blue eyes. This is due to the fact that the newborns still lack the pigment melanin. It protects the eyes from UV radiation and changes the eye colour.

Earrings for better eyesight!?

Pirates believed that earrings could improve eyesight. The idea may seem very confusing to us today. However, even in acupuncture, a treatment method that uses needle pricks at specific points, the earlobe is the point that is supposed to contribute to better vision. Nevertheless, wearing earrings does not help to improve vision.

Sneezing only works with eyes closed

Have you ever tried sneezing with your eyes open? You won't succeed because the sneezing reflex serves as protection against dust, pollen, viruses or similar and the body wants to prevent foreign bodies from entering.

Iris scans as the fingerprint of tomorrow

In some mobile phones, "Face ID" has already replaced the fingerprint for unlocking the screen. Face ID captures many facial features. However, the iris alone has a much more complex structure than the fingerprint. A fingerprint has about 40 biometric features, whereas the iris has about 260 features. Moreover, the iris hardly changes after adolescence, but the fingerprint does. Since the skin of the fingers comes into contact with many things and is thus stressed and occasionally injured, the fingerprint can still change over the years.

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