Look at this face, what do you see? An old witch or a young lady?

This picture is one of the well- known optical illusions.

Before I explain why we see this I will first explain to you, what an optical illusion is.
An optical illusion is nothing more then your brain not being able to distinguish one thing from another. Your eyes can only focus on one thing at the time. If you try to focus on 2 things you will either hurt your eyes, focus on something in the middle or you won't be able to at all.

A simple example, close one eye, it doesn't matter which one. Hold up your index finger a few inches from your eye. Now put something (like a lamp or a closet) in the background and focus on your finger. The background will look blurred. Now focus on the thing in the background and you finger will get blurred. This is because the light reflected by those objects come from different distances, the closer the object, the steeper the light comes into your eyes, the more it has to be bend by the eyes lens.

The same thing happens when you look at the woman above, not with the distance of the object but with the lines and shapes you see in your brain. Your brain can only focus on one object at the time. If you see the old witch, you won't see the young lady and vice versa.

Now look at this picture, what line is the longest?

Well, you guessed it, they are all the same length (8 centimeters).

But why do we see them as if they had different lengths? Our brains as stated before can only recognize 1 shape at the time. This is affected by the shapes and lines surrounding and being the object. If you look at those lines you can't miss the arrowheads. The first are pointed inwards the next to the same side and the last outwards.

Pointing the arrows outwards creates the illusion in your brain that the line is longer, because the arrows pointing outwards are making it longer. The total thing is longer and so it seems as if the line in the middle is longer too.

Now a totally different thing, look at this image:

Is that face turned inwards or outwards? To us it seems to be turned outwards, but if you look closely you will see it is actually the inside of a mask.

Why do we see it as if it is the front of a face? Because our brain doesn't recognize a face as being turned inwards. Our brains were made to recognize people and their faces. When we see a face we connect it to a person, a voice, a smell. Recognizing faces is crucial for our survival (instinct). This is so build into our brains that when we see a (inside-out) face we try to recognize it and we don't see it is actually the inside of a face. Besides that, seeing the inside of a face is of course impossible and in fact something we can't really imagine.

Another one of these examples:

Video.

Is the carton dragon really moving it's head?

Check out how it works!

Now, load this video and watch it (in full-screen).

Opt. Illusion Video.

You will see that at the end the moon is zooming out, or so it seems. If you look at it 'normally' this doesn't happen.

This is because the braincells responsible for noticing and calculating the distance and speed of an object are worn out by the circling 'hypnotic' circle. When you see an image after watching it for a few seconds it will seem to be zooming out because those braincells have to come to 'strength' before you can actually see straight again.

For the last example of optical illusions i would like you to click this link:

This link.

As you can see, the 2 blocks appear to be moving, but when the gray lines disappear they go at the same speed. How is this possible?

The contrast makes your brain see the blocks making 'footsteps.' Because the contrast between the blue and the dark gray is a lot less then the yellow and the dark grey, you can see the yellow better and that seems to be moving at the same speed all the time. The blue is seemingly slower because you can't see it as good as when it is above the darker gray background. For a better explanation on this check out the explanation under the flash game or whatever it is.

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Roland Wouters 2007