A thin layer of menthol between a pair of glass slides is seen magnified on the screen. The menthol is melted by heating the slides and then watched as it cools and solidifies into crystals.
The size of menthol crystals formed can be altered by changing the speed of cooling. The crystals appear coloured because they are between "crossed" polaroid screens.

When you press the red "melt" button, an electric current flows through a junction between two metals (antimony (Sb) and bismuth (Bi)) and heats it up. The Sb-Bi junction is fixed to the slides on either side of a thin layer of menthol, which melts when it reaches 43°C.

When you press the blue "rapid cooling" button, the electric current is reversed, the junction between the two metals cools down instead of heating, so the menthol freezes solid again. (Freezing Point is the same temperature as melting - about 43°C.)

If the menthol cools quickly, the crystals are tiny. If it cools slowly, the crystals grow larger.

The crystals are being viewed between "crossed polarisers" in the same way that geologists look at thin slices of rock to identify them. The colours produced by different minerals can be recognised, as well as the characteristic shapes of their crystals.

Crystallisation and melting alternates and lasts about 1-2-minutes (each).

Most crystals (including menthol) are "doubly refracting", which means that they split any beam of light passing through them into two beams which travel at slightly different speeds through the crystal. Because the two beams travel at different speeds, the light waves in each beam get out of step with one another.

White light is a mixture of colours of light, each colour having a different wavelength. Some of the colours in the original white light disappear because their waves in the two beams became completely "out of step" with one another. This leaves the remainder of the spectrum, which generally produces a very pretty mixture colour. Which bit of the spectrum is removed depends on the thickness of the crystal and the orientation of the crystal to the polarised light which enters it . Crystals which have grown in different directions thus show different mixture of colours.

The waves in the two beams in the crystal are, in fact, polarised at right angles to one another. This means that if the waves in one beam are vibrating north to south, they vibrate from east to west in the other. They then have to pass through another polarising screen (the analyser) to be recombined to see any colours. There is also a thin slice of another crystal in the way of the light to bring the colour differences between the various menthol crystals into the middle of the spectrum.