Key concepts
Animation
Perception
Optical illusions
Human behavior

Introduction
Have you ever you have ever been tricked by an optical illusion? Optical illusions can be fun, but they are also quite scientific. In this activity you will investigate the phenomenon of apparent motion by making your own flip-book animations.

Background
When you perceive an object visually, an image of that object is projected onto the back of your eye (specifically, the retina). This information is sent to the brain (via the optic nerve) where it is processed and interpreted, forming the image that you see.

One optical illusion that has had a huge impact on our culture is the illusion of apparent motion. This might sound complicated but you experience this illusion every time you watch cartoons. You perceive that the characters on the screen are moving, but in fact you are seeing many still pictures flashing rapidly before your eyes. How does this work? Basically, our brains are trying to make sense of what we are looking at by “smoothing out” the individual images that make up a cartoon into one continuous but changing scene. To do this it is thought that our brains put all of the pieces together and fill in blanks, or missing images, based on what we have already seen. Thus, our minds create the illusion of movement. Our brains not only do this when we're watching cartoons or movies—they actually do this all the time, such as when we blink.

Materials
• 25 lined three- by five-inch index cards
• Scissors
• Two binder clips
• Colored markers, highlighters, pencils or crayons
• Volunteers

Preparation
• Carefully cut the lined index cards in half to make 50 half-size cards (three by 2.5 inches). Try to cut the cards as evenly in half as possible.
• Make two stacks of 25 half-size index cards each.
•   On one of the stacks draw a small circle on the top card on the far right end of the top line. On the next card in the same stack, draw a similar-size circle just as far to the right, but on the next line space down. Continue drawing circles on each card, on the line space right below the previous circle, until you reach the bottom of a card.
•   Then on the next card draw a circle (just as far to the right) on the next line space up from the previous circle, and continue this until you reach the top of a card.
•   Repeat this entire pattern (making a series of "animated dots" going up and down along the right edge of the cards) until you reach the end of the stack. How do you think this flip-book will look when assembled and flipped?
•   On the second stack again draw a small circle on the top card on the far right end of the top line. On the next card in the stack, draw a similar-size circle just as far to the right, but four line spaces down (skipping three line spaces). Continue drawing circles on each card, making them four line spaces down from the previous circle, until you reach the bottom of a card.
•   Then, on the next card in that stack, draw a circle (just as far to the right) four line spaces up from the previous circle (again skipping three line spaces), and continue this until you reach the top of a card.
•   Repeat this entire pattern until you reach the end of the second stack. How do you think this flip-book will look when assembled and flipped?
•   Neatly stack each deck. Tap the circled edge of each card stack on a flat surface so that none of the cards' edges are sticking out (or in) from the rest in the stack.
•   Clip a binder clip on the opposite edge of each stack. Flip through both flip-books to make sure nearly every card shows while flipping. Make adjustments (such as by re-tapping the cards' edges) if needed.

Procedure
• Ask a volunteer to flip through the first flip-book, the one in which no line spaces are skipped between cards. To the volunteer, does it look like the circles move to the bottom of the card, then back to the top, and so on, going up and down along the edge of the stack? Or does the circle appear to move differently, such as by jumping around or flashing?
• Ask the volunteer to flip through the other flip-book. To the volunteer, does it look like the circles move to the bottom of the card, then back to the top, and so on, going up and down along the edge of the stack? Or does the circle appear to move differently, such as by jumping around or flashing?
• Have other volunteers flip through each flip-book. For each flip-book does the volunteer think the circles move to the bottom of the card, then back to the top, and so on? Or do they think the circle is doing something else?
• How do you think the volunteers' answers are related to apparent motion or how we perceive motion?
• Extra: In this activity you tested whether the distance between objects has an effect on apparent motion, but there are other variables you can test, such as the speed at which the flip-books are flipped and the size of the circles. How does flipping the cards in the flip-book faster or slower affect your results? Does using larger or smaller circles alter your results?
• Extra: In addition to flip-books there are many other types of animations, such as thaumatropes, phenakistiscopes and zoetropes. Look into these other animation types and how to make them. How is the spacing of objects (which you investigated in this activity using flip-books) important when making one of these other types of animations?
• Extra: You just created flip-books using circles but you can create much more complex animations with flip-books. Can you use what you learned in this activity to create a more detailed animation?

Observations and results
In the first flip-book (which skipped no line spaces), did it look like the circles moved to the bottom of the card, then back to the top, and so on? In the second flip-book, did the circles appear to move differently, such as by jumping around or flashing?

When no lines are skipped, the circles in the flip-book should have clearly appeared to move down, then up, then back down again. Our brains can easily fill in some blanks and make sense of this motion when such a relatively short distance is skipped between images (for example, cards). In the second flip-book, however, three lines were skipped between images and this was probably too great a distance for the brain to decide the motion should be a smooth up-and-down one. Instead, the circles may have appeared to jump around or even flash, especially if not every card was viewed when flipping. In other words, the brain perceived apparent motion with the first flip-book, but most likely did not perceive this phenomenon with the second flip-book.

More to explore
Experimenting with Simple Animation Devices , from Brian Wesley Rich's Science Web Site
Development of Stop-Motion Animation , from Stop Motion Animation
Perception in Animation ( pdf ), from the University of Southern California Cinematic Arts
Apparent Motion and Animation , from Science Buddies

This activity brought to you in partnership with Science Buddies