ABSTRACT
The value of the refractive index of any transparent material is a function of the wavelength (color) of the light. In general, in the visible spectrum, the index of refraction increases with the frequency. The chromatic dispersion of glasses in the spectral range not including absorption frequencies can be represented by several approximate expressions. The axial chromatic aberration contribution of a thin lens may be found by using the expressions in the preceding section. However, it is simpler to obtain it directly from the thin-lens relation. One conclusion is that an achromatic thin-lens system can be made only with two glasses with different Abbe numbers. Newton has been sometimes criticized because he said that an achromatic lens could not be constructed. The axial chromatic aberration may also be corrected with only one type of glass if either two separated lenses or a thick lens is made.
