Key Concepts
Optical illusion

Halloween is a time for sharing ghost stories and watching spooky movies. But have you ever thought about the science behind some of these uncanny experiences? Haunted houses, for example, take advantage of the way your brain uses sensory information. Often they include dim lighting and confusing sound effects to keep you disoriented and jittery, primed for fright. In this activity, you'll create an optical illusion and learn that your eyes can play some eerie tricks on you—even in broad daylight. You'll also discover this peculiar apparition has a simple scientific explanation.

Your eye has many different parts, including a light-sensitive retina at the back of the eyeball. The retina collects light signals via special cells called photoreceptors (photo comes from the Greek word for light). Your retina's photoreceptors fall into two groups: rods and cones. Rods perceive changes from light to dark and cones receive color signals. Much like the receptors in your skin, taste buds in your mouth or hair cells in your ear, your eyes' receptors collect visual information about the outside world and send it to the brain to be decoded and interpreted.

•    Several sheets of white paper
•    Brightly colored markers

•    Draw a ghost (or other shape of your choice) on one of the blank sheets of paper on the center of the page using a colored marker. Make sure your outline is bold or fill the image in with color.
•    When your drawing is finished put your picture side by side with a blank sheet.
•    Stare at the center of your drawing for 20 seconds without blinking—it's okay to count aloud to 20 or ask a friend to time you.
•    After 20 seconds immediately look at the blank sheet of paper. What do you see?
•    Take another sheet and draw another shape, this time using a few different colored markers.
•    Again, set your drawing beside a blank sheet of paper, stare at the image for 20 seconds then look at the blank sheet. Now what do you see? Do you notice any unusual colors?
•    Return to your first ghost drawing. Set it beside a blank sheet and this time stare at the center of the image with just your right eye for 20 seconds.
•    Keeping your left eye closed, use your right eye look at the blank sheet. What do you see? Now quickly close it and open your left eye to look at the sheet. What do you see this time?
•    Switch back and forth between eyes. Do both eyes see the same image? Then repeat the previous step, this time starting with your left eye.
•    Extra: Repeat this experiment using both eyes and time how long it takes for the ghostly image to fade away. Try it again, but stare for more or less time. What do you notice about the image if you stare for more time? What happens if you stare for less time?

Observations and results
Did you see a ghost or shape when you looked at the blank sheet of paper? Did you notice that colors inverted to their complement—red became green, blue turned orange, etcetera?

Illusions can be spooky—especially if you have not learned the science behind them. But they also reveal how your eyes—and brain—work together to process visual information. When you use just one eye at a time, you probably noticed that only the eye that had been staring could see the ghostly illusion. You may have also observed that the ghostly image appeared in different colors. Why was that? The image you saw on the blank paper is called an afterimage. When you stare at an image for a long period of time, the cones in your eye become overstimulated and lose sensitivity. So when you're looking at, for example, a red ghost, your red-sensitive cones become very active and eventually—if you don't blink—exhausted. When you then look at the blank page, your eyes are still receiving many different color signals. But instead of your cones sending equal signals and balancing out to perceive a white page, your tired red cones don't send a signal whereas the blue and green cones do, leaving you with an aqua-green afterimage.

More to explore
The Neuroscience of Yorick's Ghost and Other Afterimages from Scientific American
The Ghost Hand Illusion from Scientific American MIND
Sight (Vision) activities from Eric H. Chudler, Neuroscience for Kids
Are Your Eyes Playing Tricks on You? from Science Buddies
Color Aftereffect from IllusionWorks