# What Is Persistence of Vision?

### Problem:

How can we use what we know about persistence of vision to make an image appear out of thin air?

### Materials

• Friend or adult helper to observe with you
• Slide projector (a digital projector or overhead projector will work just as well). Try borrowing your teacher’s (with permission, of course!).
• An easily recognizable image to project
• Moveable white screen or poster board
• Wand (if you don’t have one, you can use a yard or meter stick, ruler, or pencil)

### Procedure

1. Set up your screen about eight feet away from whatever projector you were able to round up. Ideally, the screen should be placed in front of an open door or window so that no image will appear behind the screen when you take it away. This will likely involve turning your teacher’s projector in another direction than where the classroom’s screen is usually located.
2. Turn on your projector, and focus the image on your screen.
3. Now, remove your screen.  Where do you think the image has gone? How do you think we can bring it back with our wand?
4. Position the wand horizontally at the bottom of where the screen was.
5. Using your wrist, move the still horizontally placed wand rapidly up and down over the area where the screen used to be. Have your friend watch the wand.
6. After you finish observing what happened, make a hypothesis as to what you think will happen if you move the wand forward and backward instead of up and down. Make sure you explain your reasoning.
7. Test your new hypothesis using the same procedure.
8. Would the wand work as well if you waved slowly?  Write a hypothesis and try it.
9. Next, think about what would happen if you waved the wand at an angle, or made a circle with it. Write a hypothesis and explanation for each, and test each hypothesis.
10. Think of other ways to move the wand.  Make sure you to predict what you think the image will look like in each situation!  Don’t worry if what you see isn’t what you expected, because being surprised by what you discover can be the most fun part of science.

### Results

Waving the wand quickly up and down causes the entire image to appear in the empty space where the wand was. When the wand is waved in a sideways direction, only a small slice of the image appears on the wand.  Waving the wand slowly produces a similar effect.  Most likely, you would only see a few parts of the image, at best. Waving the wand at an angle warps the image, making it look like it was projected on a tilted screen, while waving the wand in a circle probably made the image look like it was being projected onto a three-dimensional cone or cylinder.

### Why?

Even though you removed your screen, the light from the image you projected is still focused at the same point in space.  All that you need to see the image again is a surface placed in the same spot to reflect the image’s light back to you or your friend’s eyes.  The wand reflects light back to your eyes in narrow slices. Luckily, your eye “remembers” each slice for 1/30 of a second.  This is called persistence of vision. The images our eyes pick up don’t disappear immediately after we see them; they linger!  If you move the wand fast enough, your eyes have time to send a bunch of these slices to your brain at the same time. Our brain can puts these slices together, and so we can see the entire image.

Basically, moving the wand rapidly up and down made a new screen. When you moved the wand sideways, only one slice of the image is reflected to your eye, so you did not see the whole image. When you waved your wand at an angle, you created a slanted screen, and when you waved it in a circle, you created a curved screen.

Animation relies on persistence of vision. If you’ve ever made your own flip book, you’ll know why. Take a notebook and draw a dot on the left side of one page.  On the next page, redraw the dot just a little more to the right.  Keep doing this on each page until your dot image has moved all the way to the right side of the page.  Now, flip the pages of notebook quickly. Since your eye retains each image of the eye for 1/30 of a second, the dot seems to move across the page smoothly as long as the pages are flipped quickly enough. All animation from Mickey Mouse to Monsters University relies on persistence of vision to make the illusion of movement look smooth and natural.

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