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The Disappearing Glass

We see objects only when light bounces off them or bends through them. If an object doesn’t change the direction in which light is moving, we can’t see it.

When light passes from one medium to another, the light changes speed, and when it changes speed, it changes direction a little bit. The degree to which the light changes direction is called the Index of Refraction.

You can see that light changes direction when it moves from water into air – when you look into a swimming pool, people’s legs look like they’re at a funny angle. A pencil in a clear cup of water will also appear to bend.

 

In this magic trick, the oil and the small glass beaker have the same index of refraction. That means that light doesn’t change speed or direction when it passes from the oil to the beaker and back. Since the small beaker does not bend the light, we can’t see it!


Vocabulary:

  • Refraction: The change in direction (such as bending bending) of a wave due to a change in its speed. This is most commonly seen when a wave passes from one medium to another at an angle.
  • Index of Refraction: The degree to which light is bent as it passes from one material into another.

Objectives

  • Explain the importance of observation when doing science.
    Describe how the index of refraction of light can explain why something becomes invisible.

Materials

  • A 400mL beaker
    A 250mL beaker
    Wesson™ cooking oil

Key Questions

  • What is happening when the small beaker disappears? Why do you think the glass disappeared? Why can we still see the first beaker?

What To Do

Note: Always test this demonstration first, as some glass beakers won’t “disappear.”

Preparation:
Assemble the beakers and the cooking oil on a table where all the students have a clear view. Oil can get slippery and messy, so make sure to cover the surface for safety and a quick cleanup. 

Instructions:

  1. Place the 250mL beaker inside the 400mL beaker. 
  2. Pour Wesson cooking oil into the larger beaker, and watch as the 250mL beaker “disappears”!

Extensions

  • What do you predict will happen if you put other materials (such as a pencil or straw) into the oil? Try it and observe what happens. What does this tell you about the beaker and the oil? Will all kinds of containers “disappear” in the oil? Why/why not?

Other Resources

Science World Resources | Full Unit | Magic
TELUS World of Science | Puzzles and Illusions