Speaker: Leon Poladian Repeating patterns are very popular in art, from the mosaics of the Alhambra in Spain to the mind-boggling designs of M. C. Escher. Periodic structures also occur in nature and cause stunning iridescent colours such as those on peacocks, beetles and butterflies when these patterns are the same size as wavelengths of light. This talk is an abbreviated repeat of a recent Sydney Science Forum public talk.