Optical Illusions

The eye is made up of two different types of light receptors called cones and rods.  Cones are necessary for color vision and to see well in bright light.  Rods are necessary for seeing in low light.  The two optical illusions experiments we’re about to show you fool the light receptors in the eye so the eye sees something that is not actually there.

Benham Top

The first is called a Benham Top.  In 1894 C.E. Benham created this toy he called the “Artificial Spectrum Top”.  As the black and white disk spins the eye picks up colors.

First print out the Optical Illusion Disks.

Optical_illusions_disks_2

Get an old CD and a penny.  Heat the penny in a pan on the stove (or over a bunsen burner or torch) until it is very hot.  Using pliers grasp the penny and slide it into the hole of the CD at a 90 degree angle to the CD surface.  Be careful to get it even in the middle and very straight.  Your CD can spin on this disk.

Carefully cut out the Benham disk circle and color in one half of the template with a black marker.  Darken the arcs in the other half so they are thick black lines.

Tape or glue the paper Benham disk template to the CD.

Spin the CD top you have created.

Observe the colors that appear.

Try spinning the top in low light and bright light.  Try spinning it at different speeds.  Try spinning it clockwise and counter clockwise.  Try making a different black and white pattern on the paper.

One of the most interesting things about the Benham top is that after more than 100 years science still can’t explain why this optical illusion works–we just don’t know enough about the eye.

Disappearing Colors

If you printed out the Benham Top, you also printed out the Disappearing Colors Top, but here it is again with its friend Benham. Optical Illusion Disks

In this experiment the colors spin so fast that the eye can’t separate them.  And when the rainbow colors are combined they make white.

Paint each of the top sections in bright pure color as indicated on the disk.  I recommend using water colors as they dry quickly and are easy for young kids to manipulate.  If your kids haven’t done watercolor much show them how to dip their brush only once into the water, then to work the water into the paint for 30 seconds so they get nice bright color without too much moisture.  Also remind them to wash their brush and not mix the colors in the paint tray.

Carefully cut out the circle once the paint is dry.

Cut out an identical circle from cardboard, as from a cereal box. (use an old CD as your template to trace)

Glue your colored disk to the cardboard.  Poke two holes in the disk, clear through.  Thread string through the holes and tie the two ends so you have a continuous loop of string.

Wind the string up, while holding the two ends of the string.

Pulling rapidly back and forth on the string causes the disk to spin.

Observe what you see in the colors.

Try this again, but use only two colors or three colors. Try it with different shades of these colors.  What happens if you don’t put the colors in rainbow order?

Additional Layers

  • Talk about fractions. Each of these disks is divided into fractional parts.  Name them.
  • The colored disk is in the colors of a rainbow.  Talk about how a rainbow is divided light.
  • Toy tops have been around for thousands of years and appear in nearly every culture.  They’re not always just toys for children either.  Tops have been used for gambling and prophesying.  Learn more about the history of tops.
  • Bullets shot from a rifled barrel spin like a top, but have no axis upon which they rest (they fly through midair).
  • A gyroscope is a top with two axis (points where it contacts another surface) upon which it rests.
  • A top used in a game to determine play is called a spinner.
Tagged , , , . Bookmark the permalink.

One Response to Optical Illusions

  1. Karen says:

    Very cool experiments and a very cute little boy!

Leave a Reply

Your email address will not be published. Required fields are marked *