Used contact lens. Coloured scanning electron micrograph (SEM) of contaminants on the surface of a contact lens after 3 weeks of use. A clump of dust and epithelial (skin) cells can be seen at lower left, along with two bacteria (blue). If a contact lens is not cleaned regularly then these contaminants can cause inflammation and infection. Magnification: x12500 when printed 10 centimetres wide.