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Scanning electron micrograph of photonic glass. These polystyrene hollow spheres are 172nm in diameter. They have the optical properties of an opal. Photonic crystals 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. Intuitively, the bandgap of photonic crystals can be understood to arise from the destructive interference of multiple reflections of light propagating in the crystal at the interfaces of the high and low dielectric constant regions, akin to the bandgaps of electrons in solids. The periodicity of the photonic glass structure must be around half the wavelength of the electromagnetic waves to be diffracted. This is ~350 nm (blue) to ~650 nm (red) for photonic crystals that operate in the visible part of the spectrum, or even less, depending on average index of refraction. Photonic glasses are attractive optical materials for controlling and manipu...