Composite materials are formed by combining two or more materials. Concrete reinforced with steel wires or polymer fibers is an example of a composite that is strong and has more tensile strength than its components alone. Superconductors made of metal alloys or ceramic metal oxides are used in bullet trains and MRI machines. Fiber optic cable transmits data and is made of glass or plastic fibers surrounded by glass cladding. Fiber glass combines glass fibers with plastic to create a strong yet low density material used in boats, helmets, and tanks. Photochromic glass changes darkness when exposed to light and is used in lenses and windows.