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How do different lens materials help correct chromatic aberration?

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Chromatic aberration occurs because the refractive index of glass varies depending on the wavelength of light, causing different colors to focus at different points. Because the refractive index is lower for red light than for blue or violet light, a single lens cannot bring all colors to the same focal plane, resulting in blurred or color-distorted images.123

To correct this, optical designers use a combination of different lens materials to create an achromatic lens. This process involves pairing two specific types of glass:42

Crown glass: A strong lens made of low-dispersion glass.12
Flint glass: A weaker lens made of high-dispersion glass.12

By combining these materials, the dispersive effects of one lens are offset by the other, allowing the system to bring different wavelengths of light to a common focal point. This technique, discovered by Chester Hall in 1729, is a standard method for ensuring high-quality imaging in optical systems.142