Formation of Images
Light, Sound and Waves

Seeing Circles

Enrichment Activity for 5-11 11-14

What you need

  • Blu Tack
  • String about 1 metre long
  • Two 2p coins
  • Old pair of sunglasses


  1. Stick the 2p pieces together with Blu Tack and tie the string to it to make a pendulum. Watch from about 2m away as it’s swung from side to side in a straight line
  2. Break the sunglasses in half at the nose, and put a lens in front of one eye, but look at the moving pendulum through both eyes. Swap the lens to the other eye and watch again

Results and Explanation

You'll see the pendulum moving in a circle or ellipse, instead of a straight line. When you put the lens in front of your other eye, you’ll see it moving in a circle in the opposite direction.

Sunglasses block some light, delaying signals from your eye to your brain. Normally you don’t notice, but when you cover only one eye, that eye sees the pendulum delayed with respect to what the other eye sees, so your brain thinks it’s moving in a circle.

These experiments have not been specifically safety tested for home use but we believe them to be safe if the instructions are followed. Adult supervision or direction is recommended as appropriate. All experiments are carried out at your own risk.

Formation of Images
can be represented by Ray Diagrams
is used in analyses relating to Lens
has the special case Real Image Virtual Image
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