Achromatic lenses are used to solve the problem of chromatic aberration which occurs when lenses separate white light into its component colors. Until the early 1700’s it was believed that this problem was unsolvable and that lenses would always produce this aberration. Astronomical observations were hindered by this effect. The only solution seemed to be using mirrors instead of lenses. However, the development of the achromatic lens allowed precise refracting optical instruments to rival reflecting ones.

Chromatic aberration is caused by two properties of light: refraction and dispersion. In a vacuum, light travels at 299,792,458 meters per second. However, when light enters another medium, such as water, glass, or air, it travels more slowly. Each material has a specific index of refraction that is found by dividing the speed of light in a vacuum by the speed of light in that material. Some example indices of refraction are: Air 1.00029, Water 1.33, Crown glasses 1.52-1.62, Rare earth flint 1.7-1.84, and Diamond 2.417. The greater the index of refraction, the slower light travels in that material.

Refraction is the property of light that allows lenses to work. Light bends its path when it travels between two materials with different indices of refraction. This is how lenses operate: they bend light that enters them and focus it depending on the curvature of their surfaces and their refractive index.

The refractive indices of materials are not the same for all wavelengths of light. Because of this, different wavelengths are refracted at different angles, an effect known as dispersion. In most materials, blue light is refracted at greater angles than red light. This is the property that allows prisms to separate white light into a rainbow. However, lenses also separate the different colors of light, causing chromatic aberration. Due to the different indices of refraction, each color comes to focus at a different point.

This decreased the detail that early astronomers could see with their refracting telescopes. One method astronomers used for decreasing this aberration was to increase the length of the telescope, sometimes to lengths of 150 feet. However, the problem of chromatic aberration still persisted.

In 1733 Chester Hall was the first to develop a solution. His method used two lenses that fit together to form a single achromatic lens. One lens was made to have a higher index of refraction than the other. Blue light entering the lenses would first be refracted away from the red light, and then towards it. In contrast, normal lenses refract blue light away from the red light twice. When the curvatures were adjusted properly, all colors of light were able to be focused to a single point.

However, Chester Hall did not develop his idea any further than constructing a single achromatic lens. It was not until John Dolland heard of the idea in 1758 that the achromatic lens was rediscovered, allowing chromatic aberration to be drastically reduced to acceptable levels.

*Pictures from:
Tung, Brian. "The Color Purple, Is chromatic aberration an unavoidable flaw in refracting telescopes?". The Astronomy Corner. March 2003. 3 Jan. 2004. <http://astro.isi.edu/games/chromatic.html>