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).
Blue symbolizes hope, the clear sky, and the streams of the south. Yellow stands for the sun and the Sahara Desert. Red represents unity, prosperity, and national sacrifice. Effective date: 6 November 1959.