Sugar Water Rainbow

Is sugar water colourless? No! It can be colourful.

What you'll need

• 100 cm^3 measuring cylinder
  

• 66 cm^3 water
  

• 50 g sugar
  

• 2 polarizing filters
  

• 100 cm^3 beaker
  

• Stirring rod
  

• Bright white torch
  

• Optional: Bunsen burner, tripod and gauze

Instructions

1. Pour the water and sugar into the beaker. Stir constantly until all the sugar dissolves. (Optional: set up the beaker above the Bunsen burner and stir in low heat. This helps the sugar dissolve.)

2. Pour the sugar water into the measuring cylinder carefully.

3. (Two person needed) Put a polarizing filter above and below the measuring cylinder. Be careful not to spill any sugar water onto the polarizing filter.

4. Place the bright white torch on top of the upper polarizing filter. Lift the filter-cylinder-filter above a white screen and watch a colour appear! Spin the upper polarizing filter to change the colour across the spectrum.

Ummm what's happening here?

See the colour change with a twist of the finger!! When light passes through the upper polariser, it becomes linearly polarized - all the photons vibrate along the same axis. Linearly polarized light can be thought of as a superposition (combination) of two circularly polarized "strands": clockwise and anticlockwise. Sugar molecules are chiral, meaning they cannot be superimposed onto its mirror image, and they interact with one direction of circularly polarized light more. That direction of light is slowed down so the resultant polarization is circular towards the other direction. Different frequencies (colours) of light is twisted by different amounts, thus when another polarizing filter is placed at the bottom, the colour changes according to the angle!

Mr Mould explains it really nicely here: https://www.youtube.com/watch?v=975r9a7FMqc&t=208s