This is the Checker Shadow Optical Illusion. It was developed by Edward H. Adelson of the Perceptual Science Group at MIT. The interesting thing about the below optical illusion is that square A is the exact same shade of gray as square B.
One way to identify a color is by it's Red, Green, Blue values or RGB for short. When written for 24 bit truecolor R:0 G:0 B:0 is black and at the other end of the spectrum R:255 G:255 B:255 is white. The other 16 million or so colors fall somewhere in between. Squares A and B below each have the same RGB value of R:120 G:120 B:120.
So now that I said it is true you have all obviously accepted the fact that square A and square B are indeed the exact same shade of gray, right? I mean if it is on the internet it must be true. Would it be easier if you could prove it to yourself and any skeptics you might run into (like Mom and Dad or that annoying sibling). Continue on and we'll get around to proving my statements correct.
Project: Proving square "A" is the same shade of gray as square "B".
There are a few ways you can prove that the 2 squares are the same shade.
Before we continue, right click on the image and open it in a new window. Now you have a giant image to work with.
1) You can use a graphics program like Photoshop, Paint.Net, Gimp or the Colorzilla extension for Firefox browser.
My choice is Colorzilla w/Firefox. Using the eyedropper tool you can determine
that the RGB values of the grays in both square A and square B are 120-120-120.
Not good enough for you, heh? Still not ready to trust that the computer is correct or you don't have an eyedropper tool? Either way you can move on to step 2 or 3 below.
2) Cut out a cardboard mask.
By viewing patches of the squares without the surrounding context, you can remove the effect of the illusion. A piece of cardboard with two holes created in the right spots will work as a mask for a computer screen or as a mask for a the printed illusion. Holding up this mask to the image on the screen or printed paper should be enough to convince you. But if you were like my daughter nothing but this next step would do.
3) Print the image and cut out the squares.
This is another way to isolate the patches from their surrounding context. Cut out each square along the edges. Remove them. Hold them side by side. Overlap the cut out
squares. Yup they're the same shade of gray. No denying it now, is there?
Please note that I have heard that some printers have "enhancement" processing that increases the contrast of edges. This can cause the printed squares to have slightly different values of gray. I haven't run into one of these printers yet where the overlapped squares didn't look identical, but your mileage may vary.
WHY you ask? You might have come to terms with the fact that the two squares are the same shade of gray, but how does this optical illusion work? The creator of the illusion has an explanation here.