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Plastic fibres that can be tuned to reflect only particular wavelengths of light could be used to 鈥渨atermark鈥� anything from designer clothing to passports. The fibres are made by evaporating an ultra-thin layer of glass onto a sheet of clear plastic called poly(ether sulphone). The sheet is then rolled up, heated and stretched to form hollow fibres (Science, vol 296, p 510). The layers of glass and polymer reflect light internally in such a way that some wavelengths are cancelled while others are amplified, says Yoel Fink at MIT. 鈥淚ncorporated into paper or clothing, these act like optical bar codes, so you could authenticate things in a very accurate way,鈥� he says.