Findings: Science sheds light on dark

October 25, 2002

"Dark light" that has lurked unnoticed by science has been glimpsed by physicists for the first time, writes Steve Farrar.

A computer simulation has revealed intricate shifting patterns of faint colours in the dark spots where light waves interfere with each other.

The new aspects of light have been seen by a team of physicists led by Sir Michael Berry, Royal Society research professor at Bristol University, though as yet there has not been confirmation through physical experimentation.

Light waves interfere with each other when they meet. Where two peaks coincide, they add up to produce an especially large peak. Where a trough and peak from different waves coincide, they in effect cancel each other out. This produces a dark patch - like dead water on the choppy sea - that surrounds a point called a phase singularity.

Sir Michael's team has discovered that while waves of light at such points appear to be becalmed, they are in fact alive with different coloured patterns, hidden by their very low intensities.

The physicists used a computer model that combined colour theory and wave physics. This enabled them to predict theoretical shifting colours within the supposedly dark patches.

"Interference of white light produces coloured patterns because the different wavelengths in the light add and subtract differently at different places," Sir Michael said.

The research focuses on the phase singularities, where the actual phase of a wave - its position between peaks and troughs - is unknowable.

"In these special places, the phase of the wave is undefined, just as time is undefined at the North Pole," Sir Michael said.

"However, the colours hidden in the darkness can be predicted. By magnifying the intensity there, the colours form characteristic and striking patterns."

The findings are published in the New Journal of Physics .

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