In order to see, your eye must focus light on its retina, convert the light into electrical impulses, and send those impulses to your brain to be interpreted. When the electrical impulses arrive in the visual cortex of the brain, the brain "reads" them and interprets them into an image of color and light. It then flips the image (the light was projected on your retina upside down) and fills in for the blind spot if necessary. All this happens almost instantaneously, allowing you to read a book or enjoy a beautiful sunset. Generally the process works flawlessly, but sometimes the color, light, or pattern of an object can "trick" the brain into interpreting the image incorrectly, so you think you see something differently from how it really is. This is an optical illusion.
Some optical illusions happen naturally: have you ever noticed that the moon looks much bigger when it's close to the horizon than when it is high in the sky? Or have you seen what looks like water on the highway when driving in the sunshine? These are both optical illusions - the moon doesn't really change size, and the road may be dry as a bone!
Other illusions happen because of a certain combination of colors and shapes, changes in background, or because our eyes get tired. Here are just a few types of illusions. Look at our illusions example sheet to see pictures of each one.
These are illusions that have two pictures in them; which one you see depends on how you look at them. You've probably seen the ambiguous illusion that looks like either a vase or two people facing each other. In another kind shadows and contrast can make an image look either like a cube or like an empty room. Check out the illusions sheet to see some more examples. Try drawing your own ambiguous illusion!
Sometimes things look like they are moving, but they really aren't! This is a complex illusion that depends on color, contrast, and peripheral vision. (That's when you see out of the "corners" of your eye rather than focusing on something directly.) One famous example of anomalous motion is this rotating snakes image. It is not animated! Try printing it out on a color printer and it will still work. Focus very hard in the middle of the image and the motion will stop. The image to the right is another example of anomalous motion called "sea sickness." Be careful - looking at anomalous motion illusions might actually make you seasick! Look at the larger version in the illusions example sheet to see it move.
Have you ever seen something even after you've stopped looking at it? This is called an afterimage. Look at the example on the illusions sheet. Stare hard at the red ball for about 20 seconds, then look at the empty flask, letting your eyes go a bit unfocused. You will see the ball inside the flask now, but it's changed colors! How does it work? As you stare at the red ball, the red receptor cells (called cones) in your eye get tired and stop sending signals to your brain. When you look at the interior of the flask, you would normally see a white background, which is a mix of signals from red, blue, and green cones. Instead, you saw a bluish-green ball inside the flask, because the red cones were still too tired from looking at the red ball to transmit information. Try staring at the green ball now. What color is the afterimage?
In these illusions, objects look like they change shape or size. Distortion illusions happen because our brains process objects in relation to what is around them. We expect things in the distance to look smaller than things up close, for example. Distortion illusions play with those expectations to trick us into thinking we're seeing something different than what is there! Check out several distortion illusions on the illusions example sheet.
It may look okay on paper, but the minute you consider whether it can exist in real life, you'll be scratching your head! Impossible objects look reasonable, but really defy laws of perspective and geometry—outside edges become inside edges, staircases lead up and down in a continuous loop, and more.