Spooky Science: Make a Ghostly Illusion

A visual Halloween trick from Scientific American

By Daisy Yuhas

SEEING THINGS: You won't believe your eyes in this spooktacular illusion. Image: George Retseck

Key Concepts
Neuroscience
Photoreceptor
Afterimage
Optical illusion

Introduction
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.

Background
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.

Materials
???? Several sheets of white paper
???? Brightly colored markers

Procedure
???? 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?

Source: http://rss.sciam.com/click.phdo?i=d14e5cb75c36a0d177f1493d4d3d9751

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