Jeff Hecht, consultant
(Image: Alexander R. Albrecht, University of New Mexico)
The three coloured jets aren't what they seem. They look like fluids dyed different colours mixing to make a clear liquid. But all the water is clear: the colour comes from red, green and blue lasers. This photo won Alexander Albrecht of the University of New Mexico in Albuquerque first prize in the 2012 After Image photo contest run by Optics & Photonics News.
The colours seem to be flowing through the jets because an effect called total internal reflection is confining the laser beams. Each laser is aimed along the centre of a jet. As the jet bends, the light hits the boundary between water and air at a glancing angle, so it is reflected back into the water and travels further along it. If the light is to travel all the way down the jet, the surface of the jet must be smooth and even to keep the light and the water from breaking up in turbulence. Some light passing through the water jet scatters out of it by bouncing off water molecules, an effect called Rayleigh scattering.
Physicist Daniel Colladon first demonstrated light guiding along a water jet in 1841. Another physicist, John Tyndall, later repeated the demonstration in his popular lectures at the Royal Institution in London. The effect is credited with inspiring concepts from illuminated fountains to fibre optics.
The red, green and blue laser beams mix together to make white light because they are the same intensity and match the human eye's three colour receptors. Combining different blends of these three primary colours can produce the whole range of colour visible to the human eye, including colours such as pink and brown which are not in the rainbow or solar spectrum. Video displays produce images in the same way, by modulating the brightness of tiny red, green and blue emitters across the whole screen.
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