Combination of lenses made from materials of different refractive indexes, constructed in such a way as to minimize chromatic aberration (which in a single lens causes coloured fringes around images because the lens diffracts the different wavelengths in white light to slightly different extents).
Red and white are the colours of Bohemia, dating back to the 13th century. Blue represents Moravia. Unlike that of the Slovak Republic, the Czech flag is not based on the pan-Slav colours. Effective date: 1 January 1993.