Lose a Glass in a Glass


Use this activity to demonstrate the role of indexes of refraction in our ability to differentiate the appearance of different materials. Note: adult supervision is recommended for this activity because it involves paint thinner.

Required Materials


  • A clear glass jar free of labels or other marking patterns
  • A smaller clear jar that fits inside the larger one
  • Paint thinner - also known as petroleum distillate (Caution: We suggest that you use this substance only with an adult present - it is poisonous and can also burn your skin or eyes. Use in a well-ventilated place because the fumes can be harmful. Be sure there are no open flames in the room, as this substance is flammable.)

Activity Directions


  1. Put the smaller jar into the larger one.
  2. Fill the smaller jar and the space between the two with paint thinner.
  3. The small jar seems to simply disappear.
  4. When you are finished with the experiment, carefully pour the paint thinner back into its original container and seal tightly.

Here's What's Happening


The speed of light through any transparent material is always slower than the speed of light in a vacuum. Scientists have measured the speed of light in many transparent materials. They have assigned specific numbers to a variety of materials. The numbers compare the speed of light in each of the different materials with the speed of light in a vacuum. This number is called the material's index refraction. The index of refraction is also a measure of how much a material will bend light. A material that has a high index of refraction, like a diamond, will bend light more than a material with lower index of refraction, like water.

You can see a boundary line between two transparent materials only when they have different indexes of refraction. It's easy to see the edges of a glass jar in air. You see the outlines of the smaller jar when it is inside the larger one because it is surrounded by air. Paint thinner has an index of refraction that is very close to the index of refraction of glass. When you replace the air with paint thinner, the light is no longer bent at the boundary of the inner glass, and you can no longer see its outline.

To find this experiment and many more please read Light Action! Amazing Experiments with Optics by Vicki Cobb and Josh Cobb, illustrated by Theo Cobb. To purchase a copy click here.