Optical Illusions

An optical illusion is the erroneous perception of reality, or in other words : what you see is not what's there.

In short :

• Your eyes are not perfect. For example, they can get tired and they have a blind spot.
• Every image must be interpreted by the brain. Sometimes that process can go wrong.

So I just said your eyes are not perfect. Allow me to demonstrate. Look at the image below. Since the squirrel doesn't trust the all-too-friendly smile of the boy, he wants to get out. Let's make him disappear ! Close your left eye, then look at the boy (probably with your right eye). Slowly decrease (or increase) the distance to the screen until ... the squirrel is gone ! Whoa ! Seems you DO have a blind spot, and it's large, too ! There is a certain spot in the back of your eye where you can't see. It's where the optical nerves ("wiring and cables") enter the eye.
This is also a brain illusion. Think about this : do you see a huge floating gap everywhere you look ? No ? But you just learned you have a blind spot so ... your mind just guesses what should be there and shows it to you !

Your eyes are also easily tired, letting you see things that aren't there. Take this flag, for example. Stare at the white dot for 20 seconds or so, then look at a white surface such as a wall. What do you see there ? The reason for this (in simple terms) is that the light-sensitive cells are tired from the staring at those colors. In this example, the "blue" cells are tired, so you see the signal of the "red" ones better. Result : red stripes.

A lot of illusions are caused by the processing in the brain. Take this simple animation below for example : does the wheel turn left or right ? You can make it turn in any direction you want, just by thinking of it. It may help to look at the edges : look at the left edge and it turns left ... How does it work ? This wheel is not turning left or right. It consists of just two different frames, there is no "direction" involved. But your brain thinks : "It's moving, it's a wheel, so it must be turning in some direction, right" ? It just makes up the missing frames on its own.

In the previous example, our brain doesn't understand the nature of the motion. In this powerful illusion, it just adds motion where there is none ! Just look at the black spot and move back and forth to the screen. You will immediately see the circles rotate. If you move away from the screen, every shape is getting smaller. So for you, they are moving inwards. The slanted edges give you brain the impression that the shapes are not only moving away, but that they're rotating, too.

Why is it that we can be fooled so easily ? Are we humans so badly manufactured that we can't even do a simple task like looking around decently ? Another illustration to regain some respect ... At the left you can see a beautiful autumn scene. Nothing wrong with that. At the right is the information that enters your eyes ... Your brain has to merge two slightly different scenes together into a threedimensional image, invert it, fill up the blind spot and the edges from your vision (you don't see a border where your pupil ends, do you ?) and finally, it has to make sense of it. And all that while you're taking a nice walke through the woods.
It's not so strange that something this complex can be fooled sometimes.