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Through a light, darkly

October 18, 2002

A British physicist has come up with a way to reveal the shifting and shining colours that form in the dark spots where light waves interfere with each other. The patterns await experimental demonstration but computer-generated images are already illuminating new aspects of light that had until now remained in the shadows.

When two waves meet their peaks and troughs interfere. If the peaks are in phase you get a higher peak as the energies of each wave add together, two troughs take away from each other to make a deeper trough. On the other hand, when a peak and a trough
meet they apparently cancel out. For waves on the sea, the effect is a patch of dead water. For light, the picture is altogether more complicated and beautiful as Professor Sir Michael Berry, of the H. H. Wills Physics Laboratory at Bristol University, has found out.

"Interference of white light produces coloured patterns," explains Berry, "because the different wavelengths in the light add and subtract differently at different places." He has combined colour theory and wave physics to look more closely than ever before at the calm water - the dark light. They have used a computer model to simulate the interference patterns produced by two light waves. Where peak and trough meet to cancel each other out they see a region of dark light called a phase singularity. "In these special places the phase of the wave is undefined, just as time is undefined at the North Pole," says Berry. "However, the colours hidden in the darkness can be predicted," he adds, "by magnifying the intensity there, the colours form characteristic and striking patterns. These theoretically predicted colours of dark light have yet to be investigated experimentally but Berry`s work predicts many different colour patterns in the dark light.

For instance, random light waves interfere to make delicate features, like the shimmering light at the bottom of a swimming pool, where the waves appear to move faster than light. Berry has even unwoven the rainbow a little more than Newton to show that phase singularities lie at the heart of this most beautiful but transient natural phenomenon.

Institute of Physics