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How does an achromatic lense work?

A view looking up at the upper part of a large sandstone building. The view is seen twice, one upon the other. The building has large classical-style half-columns, five windows and a slate-covered roof surrounded by a balustrade-type structure. Behind the building can be seen sky and clouds. In the upper view of the building, the image is sharp and clear. In the lower view, the image is slightly blurred and a purple tinge can be seen along the right-hand edges of the building.

An achromatic lens is a combination of concave and convex pieces of glass that focuses the different colour wavelengths in light to a single plane. The concave lens is made from flint glass and the convex lens from crown glass. Each type of glass disperses the colours differently — put together they counterbalance each other and produce a sharp image.

Right: This photograph shows the same image with and without chromatic aberration — the purple fringing around the edge of the building.
Photo: Stan Zurek.

Diagrams showing a chromatic aberration and an achromatic lens

Developed for use in telescopes, achromatic lenses are now found in all kinds of optical instruments, including cameras.

Left: Chromatic aberration and achromatic doublet based on an image by an unknown artist.

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