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What You Need:

  • Invisibility-through-refractionGlass test tubes
  • 500-1000 mL beakers
  • Baby oil

Light and reflection

What Do You Do?

  • Thoroughly go over the refraction presentation.
  • Fill the beaker about 3/4 full with baby oil.
  • Put the glass test tube into the beaker with oil–can you see the test tube?
  • Now slowly pour baby oil into the test tube (while it’s still in the beaker).  Can you see the test tube disappear?

What Happened?

When light travels from one material to another it usually changes speed. This change in speed makes light bend, and our eyes can see the change.

The refractive index is a measure of the change in speed of light when it passes from one material (like water or air) into another.

This activity uses two objects that have a similar refractive index, and light does not get bent (refracted) when it passes through the baby oil and into the glass. As no light is bent, our eyes cannot see a change in what’s there.

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