Warning: This activity requires cutting with scissors. Younger scientists should get an adult to help.

You will need

• Black cardboard
• A compact disc (preferably an old one)
• Lip balm in hard plastic tube eg Chapstick, Banana Boat, etc.
• Glue
• Scissors
• Sticky tape
• Template (Download the pdf)

What to do

1. Download and print out a copy of the template. Cut out the frame and trace around its edge on a sheet of black card.
2. Cut the frame from the black card.
3. Cut the image from the template and glue it onto the frame. Make sure the top of the image sits against the bottom edge of the slots.
4. Roll the frame to form a circle with the image on the inside, overlapping and gluing the tab at the end onto the outside. Secure it with sticky tape if you need.
5. Fold the four tabs on the bottom of the frame so they point out. Use sticky tape to secure it to the middle of the compact disc (CD), with the hole in the centre.
6. Take off the lid of the lip balm plastic tube. You’ll find the tube now fits perfectly inside the hole in the middle of the CD, while still allowing it to spin. Put the CD onto the tube and use your free hand to spin your zoetrope.
7. Watch that frog jump!

What’s happening?

To understand how the zoetrope works, it’s easier to think of it in two parts – the pictures and the slots. As the disc spins, you watch the pictures moving on the other side of a slotted wall.
Because the pictures are moving by so fast, your eyes can’t see them individually, making them blur together. However, the slots help separate the blur into ‘frames’ again, allowing the eye to see a series of similar-looking pictures.
Your brain has a couple of neat tricks whenever it sees a sequence of similar images. One of these is called ‘beta movement’, which is the assumption it makes whenever it sees an image change position. This assumption says the object you’re seeing must have moved. Of course it didn’t move at all – it’s a completely different picture. But your brain doesn’t know that.
Another shortcut it uses is called the ‘phi phenomenon’, which is the sensation of movement whenever we see something suddenly appear or disappear (think of lights blinking in a circle around a classical cinema sign).
We used to think moving pictures such as these were caused by something called the persistence of vision, where your eye’s retina holds onto an image for a fraction of a second after it had disappeared from view. Yet while it is true this occurs, it’s not enough to explain what we see in our zoetrope.

Applications

Right now, your computer screen is flickering. Of course, it’s a little too fast for you to see, changing images between 60 and 85 times a second. This is measured in hertz (Hz), and varies depending on what sort of monitor you have. Your television also flickers, at about 50 Hz. Every frame shows a slightly different image, which you see as a smooth, moving picture.
In northern Italy, the world’s largest zoetrope spins at a massive 200Hz, and advertises Sony’s MotionFlow high definition televisions.
All of the movies at the cinema once came on long strips of celluloid film. This strip was made up of a series of different pictures that ran quickly over a projector, so you saw 24 of them every second cast up on a big screen. Today, many cinemas are going digital, meaning they have no need for big reels of film.
But they all still rely on your brain tricking you into seeing a moving image.

• Try another Science by Email activity investigating perception
  

More information

• How many frames per second can our eyes see?
• World’s largest zoetrope
• The phi phenomenon – can you see the moving shadow?