Answer: It is an optical phenomenon resulting in false visual impressions
Firstly, I think that the definition above doesn't even scratch the surface of the complexity of illusions, and secondly, I reckon that optical illusions are intrinsically fascinating for two reasons: (1) because they produce something that is not as it seems, which in turn makes us challenge 'face-value'; and (2) most of them are master-classes in physics and psychology (in varying degrees).
If that wasn't enough to establish the credibility of optical illusions, Bristol University actually has a 'Professor of Illusions'. Oh yeh. This is serious stuff.
"So!" I hear you cry, "what on earth has made you start banging on about optical illusions?"
Well, I was listening to the radio today when I heard a discussion on the subject, and (being the saintly soul that I am), I decided to have a scout around to find some gems to keep you entertained.
Vid.No.1. How cool is this?! Imagine the maths that went into calculating the perspective
Pic.No.1. It looks like moving waves doesn't it? I can assure you that this picture is completely still - see, just touch it (ha ha, just teasing)
Pic.No.2. This illusion is actually a chalk drawing on the pavement - a true anamorphic picture
Pic.No.3. This is what Pic.No.2 looks like when viewed at another angle (do you fancy calculating the perspective on that? Yeh, me too)
Pic.No.4. Another anamorphic picture, also drawn on the pavement. Bloody amazingly talented.
Pic.No.5. Frisbee optical illusion
The optical illusion above, apparently came about entirely by chance. Have a look at the picture, and in particular the two frisbees that you can see. Initially you might think that the picture has captured a moment when both frisbees are in the air. But that is not the case. Only one frisbee is actually airbourne.
The black frisbee is in the air, and the yellow one is on the grass. But the photographer has captured the moment when the shadow of the black frisbee is in just the right place to convince us that it is the shadow of the yellow frisbee, and that the yellow frisbee is actually in the air. How cool?
Pic.No. 6. Expanding Star
The pattern above is stationary – yet it seems to expand, especially if you don't look directly at it, for instance when reading this text. But here is a bizarre fact. There is a percentage of the population for whom these illusions don't work...... that isn't logical surely?
Pic.No.7. Chequer Board
And now for my last offering. If you look at the chequer board above, it looks like the centre is bulging. Not so! Every single one of the large black and white squares is of the same size and proportion. At first I didn't believe it either, so I got a ruler and held the straight-edge up at the screen (that's what I call technological advancement), and lo and behold, they are all the same size.
Crikey. Isn't a bit of 'random digressing' fun? Has anyone got any interesting optical illusions to add to the melee?