1) Same Color Illusion.
copyright: Edward H Adelson and web.mit.edu
Look at the image above. Which of the squares A and B do you think is the darker shade of gray? A? Well, as it turns out in every such optical illusion, the answer is - None. They are the same shade. Exactly the same shade. :) The brain interprets the intensity based on the context. Ever wondered why sunspots are dark? As in, it's not as if those regions aren't bright and hot themselves, so why do they appear dark? It's because they are embedded in much hotter and brighter regions of the Sun. This is exactly what is happening here. A seems darker because it has light squares around it. B seems light because it is surrounded by dark squares. I feel that the strength of this particular image is in the fuzzy boundary of the cylinder's shadow. This means that B's shade merges into the light square's shade seamlessly fooling the brain into expecting that B must actually be the same shade as the light squares, only that they are slightly darker due to the shadow. But same as A? No way! well, it is. :) Perhaps the image below will help in visualizing it.
copyright: Edward H. Adelson and web.mit.edu
2) Blue or Green?
Copyright Akiyoshi Kitaoka
You know what's coming. But you can't believe it, can you? The spirals that are clearly blue and green are actually neither blue nor green, but a combination (RGB=0,255,150). i.e. they are EXACTLY the same color. unbelievable? I know. This one actually makes one forget the first illusion entirely. I mean, it was difficult to believe that one, but surely this is simply impossible? Well, copy it to paint/photoshop and see the RGB values. Or, just crop some pixels from the blue and green spirals without their pink/orange context and juxtapose them. Or, if you don't really have to do it yourself to believe it, just head on to this article at the badastronomy blog and see for yourself. This really is stunning.
3) Ok, this one is fun. Look at this one. Which direction is the (apparently nude, but I digress... ) spinning? Clockwise? Counter-clockwise?
No idea about copyright, but I found it here. Anyway, once you know your initial answer, look again. Perhaps looking at the shadow of the feet will do it the first time. The spinning direction flips instantly! It is actually much easier to flip the interpretation in your mind after you succeed in seeing it once. I can do it within 5-10 seconds now, and yet it is such a rush everytime it happens! :) Specially when I focus on her face. Suddenly the hair jumps from front to the back. :) I think this is essentially the same thing as flipping the 3-D interpretation of the Necker Cube in your mind. But, somehow this spinning girl makes it much more fun! :)
4) And now, the killer. Hollow mask illusion. I don't need to say anything as they say it all in the video.
Incredible, no? I have always wanted to have one of these masks at home. Specially this next one, with einstein. But it is definitely not worth 60 dollars....
Do you have any favorite illusions? Do share in comments.