6A40.40 Variable Index of Refraction Tank


Refractive index gradients, Fermat's principle, law of refraction


Light bends when travelling through a concentration gradient of sugar in water. The top layer of liquid is pure water while the bottom layer is concentrated sugar water. In these layers, there is no change in refractive index and no bending of light. In the middle, a gradient forms.



  • [1] Plexiglas tank with sugar solution
  • [1] Green laser pointer
  • [1] Stir rod
  • [1] Roll of paper towels
  • [1] Cart

Important Notes

  • Transport the demo carefully! If transporting it yourself, allow extra travel time. The demo can withstand a slightly bumpy cart ride through the AQ (terrible corrugated floors!) and some sloshing horizontally as long as the layers don't mix vertically.
  • The demo looks best with two nights to settle, so staff need three full days of notice. It's usable with one night to settle, though the gradient is sharper.
  • Spills tend to get sticky.
  • Lasers can cause blindness. Use with caution.


  1. Turn off all the lights you can. Aim the camera at it, if applicable.
  2. Shine the laser at a small angle up, from the bottom layer to the middle layer. The beam should bend back down. The physics is that of an upside-down mirage. In the tank, from top to bottom, the index of refraction is increasing, as in a mirage from the ground up.
  3. Shine the laser pointer horizontally across the top, then horizontally across the middle and bottom. The beam should be straight in the top and bottom segments, and bend down in the middle. In a gradient, light bends towards the higher-index part, as the law of refraction predicts, or according to Fermat's principle. In a homogeneous medium (top and bottom), light does not bend.
  4. Mix up the solution using the stir rod.
  5. Shine the laser through the sugar solution at all angles, showing it no longer bends inside the homogeneous solution.


Additional Resources


  • PIRA 6A40.40


  • Don't attempt this at home!

Last revised

  • 2020


  • Additional equipment for solution prep: [1] kettle, [1] 4 L beaker, [1] bag of sugar, water, [1] index card, [1] funnel.
  • Sugar solution prep: boil ~2 L of water in kettle. Pour into the beaker and stir in sugar until it has difficulty dissolving (3 L total sugar water is too much; could try less sugar water and top up with more fresh water). Pour solution into tank on a cart. Place index card on water's surface. Pour about the same volume of fresh water onto the index card through funnel to create fresh water layer above the sugar water. Let settle for 2 nights. Also usable after 1 night if necessary.
  • After 4 nights at room temperature, the top water layer is sufficiently sugary to bend the laser beam path slightly. This may be a problem for courses that want to show a straight laser beam at the top. After (at most) 8 nights at room temperature, the laser beam path bends everywhere in the tank.

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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.