When I put ice into my whisky I see lovely swirling patterns in the liquid. These are obviously to do with temperature and density differences between the melting ice and spirit. But what mechanism allows me to see these differences? What optical effects are at play?
鈥� What your correspondent sees is caused by the mixing of liquids at different densities, but it can only be observed thanks to the refraction of light.
Light travels in a straight line through a vacuum and through a medium of uniform density. When light moves from one medium to another, its path changes direction by an amount that depends on the difference in density of the two media. The angle is calculated using Snell鈥檚 law.
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When light passes through a liquid of variable density, its path will change continually. This dynamic refraction results in an attractive shimmer effect.
To maximise this effect, allow your glass of spirit (or warm water for abstainers) to sit a few minutes so the liquid is perfectly still. Hold the glass to a bright window and gently put in an ice cube.
You will see the ice melt and the cold, dense water flow around the ice in a laminar way. The flow then becomes turbulent as it moves down and mixes with the less dense liquid below. The shimmering dynamic refraction allows you to see and appreciate the beauty of the flow patterns and the chaotic mixing.
An upside-down negative of this phenomenon can be seen if you hold the flame of a candle or cigarette lighter in front of a projector while it is shining white light onto a screen. The invisible convection currents created by hot, less-dense air rising and mixing with the denser air of the room can be inferred from the shadows cast on the screen. Don鈥檛 use a match because the mesmeric effect of the moving shadows may cause you to burn your fingers. In a still room, the shadows from the convection current can be up to a hundred times the size of the flame. Such dynamic refraction can also be seen in the heat haze above sun-soaked roads and above radiators by sunlit windows.
Dynamic refraction and its associated scintillation have even been immortalised in a nursery rhyme: .
David Muir, Science department, Portobello High School, Edinburgh, UK