Opal cubic zirconia. Coloured scanning electron micrograph (SEM) showing the cubic structure of the photonic zirconium dioxide crystals that make up an opal cubic zirconia. Photonic crystals are attractive optical materials for controlling and manipulating light flow. These regularly arranged hollow spheres measure 172nm in diameter and have the optical properties of an opal. They contain regularly repeating regions of high and low dielectric constant. Photons (behaving as waves) either propagate through this structure or not, depending on their wavelength. One dimensional photonic crystals are used, in the form of thin-film optics, in applications from low and high reflection coatings on lenses and mirrors to colour changing paints and inks. In nature photonic crystals are found, among others, in opals, bird feathers and butterfly wings. Magnification: x20,000, when printed 15 centimetres wide.