Why is glass transparent?

Materials are transparent when their electrons cannot absorb visible light. In glass, made mainly of silicon dioxide, the energy gap between the electrons’ occupied states and the next available states is larger than the energy of visible light photons. Since the photons cannot be absorbed, they pass through. Ultraviolet light, which has higher energy, is absorbed by ordinary glass, which is why you do not get sunburned through a window easily. Adding metals or impurities to glass can create energy levels within the gap, making the glass absorb certain wavelengths and appear coloured.