Achromatism

Achromatism, in optics, property of a lens system of bending a beam of white light in such a way that all its component colours are brought to a focus at the same point, thus obtaining a sharp image. A single lens cannot achieve achromatic focus because different wavelengths of light are bent by differing amounts as they pass through the lens. For achromatic focus, at least two lenses that are made of glass with different refractive indexes must be combined. This principle was discovered in 1757 by John Dollond, a British optician.