6A40.30 Disappearing Glass Tubes


Refractive index, refraction, geometrical optics


Light refracts and reflects when it reaches a medium with a different index of refraction. The index of refraction of mineral oil is almost identical to that of a glass tube, so light does not refract or reflect between the oil and glass, making the glass almost invisible in oil.



  • [2] Beaker
  • [2] Glass tube
  • [1] Water bottle
  • [1] Bottle of mineral oil
  • [1] Document camera or overhead projector

Classroom Assembly

  1. Place the beakers, each with a glass tube inside, as desired for an overhead projector or document camera.
  2. Put the bottles of water and mineral oil nearby.

Important Notes

  • After the demo, please return the mineral oil to its container for re-use!


  1. Pour water into the beaker for water. Note how the tube is visible due to refraction and reflection at the interface.
  2. Pour mineral oil into the other beaker. The submerged section of tube should almost disappear.


Additional Resources


  • PIRA 6A40.30


  • Don't attempt this at home!

Last revised

  • 2017


Related AV

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If you have any questions about the demos or notes you would like to add to this page, contact Ricky Chu at ricky_chu AT sfu DOT ca.