Magic Arcs Optical Illusion Amaze your friends as the arcs magically appear to shrink and grow email to friend
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1. Cut two identical arcs out of cardboard or a paper plate, or get two cup "sleeves" from Starbucks... unfold the sleeves and you have two identical arcs!
2. Color the two arcs each a different color, because the same color doesn't make a very amazing experiment!
3. Hold one arc directly above the other and ask your friends, "Which arc is bigger?" The bottom arc will always look bigger than the top one.
4. Now reverse the arcs to make the other colored arc look bigger. It always works! It appears as if you caused the arcs to magically stretch... but your eyes played a trick on your brain.

Presentation Ideas

Part of the fun of doing a science demonstration is making your audience ask, “How did you do that?” Here are a few ideas to consider when you’re developing your own ARCtical Illusion presentation.

1. Start by showing that the arcs are exactly the same length. Explain that these arcs are made out of a special material that can be made to “stretch” or “shrink.” Now, pretend to s-t-r-e-t-c-h the red arc. Prove that it worked by holding the yellow arc above the red arc. “See...the red arc is bigger!”
2. After showing that one arc is bigger than the other, “shrink” the bigger arc by pretending to squeeze the sides toward the middle.
3. The Convincer! Hold the arcs so that the yellow one is on the bottom and the red is on top. Of course, the yellow arc looks bigger. When someone says, “I know that trick...they’re the same size,” quickly place the red arc on top of the yellow one, but secretly allow the yellow arc to overlap the red one by about one inch.

How does it work?

This is an illusion of comparison. The psychology textbooks tell us that a segment of a circle seems much longer if it is placed under an identical segment in such a way that the ends are even at one end. The secret is that the longer outside edge of the bottom arc is being compared against the shorter inside edge of the top arc. When the inside curve of the top arc is placed next to the outside curve of the bottom arc, the bottom arc always appears to be bigger. The illusion is even convincing when the top arc is centered over the bottom arc.

Stretching Bananas

Bob Friedhoffer is a world famous magician and science writer who offered this clever little piece of optical foolery. Instead of using the arcs, you’ll need two bananas that are identical in size. Hold one banana above the other just as you did with the arcs. Which banana is bigger? The bottom one...of course! Bob suggests that you pretend to stretch the smaller banana to make it equal the size of the bigger banana! It’s the great stretching banana trick.

So, the next time you’re at the grocery store, hold a few bananas up pretending to compare their sizes. When someone asks what you’re doing, tell them that you’re trying to find two bananas that are the same size. “The bottom one looks bigger than the top banana, doesn’t it?” Out of frustration, pretend to “stretch” the top banana. After the stretch is complete, place the bananas on top of each other to show that they are now equal in size. You’ll know that you’ve fooled someone when you see them trying to “stretch” bananas on their own!