We have already seen, in the previous chapters, that the optical properties of a lens depend on the refractive index of the glass; this refractive index is a function of the wavelength of light and, for all transmitting materials, the refractive index varies in the manner indicated in Fig. 5.1. The effect of this variation of refractive index, known as dispersion, is to produce a wavelength-dependent variation of optical properties, known collectively as âchromatic aberration.â Sir Isaac Newton invented the reflecting telescope because he considered this aberration impossible to remove since all glasses have dispersion. Fortunately for the development of modern optical instruments, in 1729 Chester Moore Hall, a barrister, proved him wrong. He âmadeâ an achromatic doublet by combining a positive-powered low-dispersion (crown) glass and a negative-powered high-dispersion (flint) glass. He realized that this would work as required by experimenting with prisms until he found a pair of prisms, using different glasses, which produced deviation without dispersion. He reasoned, correctly, that the same principle would work with lenses.
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