Optical Illusions
Optical illusions make you perceive something as different from reality. You may “see” something that is not there, or you may perceive things that are impossible in reality. Here are some fun ones from http://kids.niehs.nih.gov/games/illusions/:
With this one, if you follow the directions, it looks as if the circles actually rotate.
This is an object that is impossible to build, though it can be drawn in 2-D:
With this one, you may perceive the person at the bottom left of the image as holding a line for the person on the top left of the image to hang on to as he hangs over a railing. You may perceive the person at the bottom right of the image as climbing a ladder to reach the people at the top right of the image, who are standing on a balcony. Yet both situations cannot happen at the same time!
In this painting, Waterfall by M.C. Escher, in what direction is the water flowing?
In this artwork, Sky and Water I by M.C. Escher, where does the ocean stop and the sky begin?
With this one, you could see either a man or a woman:
In this one, are there 2 columns or 3? It looks like 2 if you stare at the top of the structure, but 3 if you stare near the ground:
Here is a painting by Bev Doolittle called The Forest Has Eyes. Depending on how you perceive it, it could be a normal forest scene, or there could be multiple faces looking back at you:
Here is a link that points out 13 faces.
Optical illusions happen because of our automatic perception mechanisms. For example, we divide an image into figure and ground; we cannot see both as the subject. Also, when looking at a 2-D image, it is hard not to perceive it as it would be in 3-D, where laws of the physical world apply. So, that is why some optical illusion scenes look impossible. With images such as The Forest Has Eyes, what you see can depend on if you look at individual details (e.g. a rock), or the overall scene.