Photonic crystal diffraction pattern. This inter- ference pattern was formed by passing orange laser light through a photonic crystal. The symmetrical light and dark bands indicate a regular crystal structure. Three dark Kossel lines are seen around the bright central area, which are characteristic of the interference pattern of a three-dimensional structure. The structure of a photonic crystal prevents specific frequencies of light from propagating in it. In such a crystal, light can be guided in very small spaces with no loss of information. Photonic crystals may find uses in optical computers.