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7.8.2 Dispersive power

Dispersion is important in the selection of materials for lenses as it provides a measure of how different wavelengths of light are separated. White light comprises different colours of light from the electromagnetic spectrum, from red through to violet (and all the rainbow of colours in between). The different colours have different wavelengths, and ideally you do not want these to separate as they are transmitted through the lens, or objects will appear to have coloured haloes around them. The degree to which a material disperses light (refracts different wavelengths by different amounts) is known as the dispersive power, D. So for lenses, a low D is desirable.