Countless Variations: Lens Combinations
All About Glass
The world began to realize that so far it had only toyed with glass. Now a brand new material was born.
-Walter Kioulehn, Odyssey of the 41 Glassmakers, 1959
By the mid-1800s, there were still only two kinds of optical glass: soda-lime crown glass and lead-containing flint glass. Opticians doubted they’d ever have more choices. Then a German glass chemist made a crucial discovery. In 1884, Otto Schott proved he could precisely alter the optical properties of glass by changing its chemical makeup. The variations in glass, it now seemed, could be endless.
What sparked Schott’s discovery was a plea for help published by a German physicist, Ernst Abbe. No matter how Abbe had configured microscope lenses, the image wasn’t clear at high magnification. A colored fringe blurred the edge. The fringe, caused by chromatic aberration, occurs because a lens can’t focus all colors of light to the same point. The only solution, Abbe had concluded, was to change the glass itself.
Schott sent Abbe samples of glasses he had made while experimenting with new recipes. After 93 tries, they found the right composition. Today, optical designers can correct for chromatic aberration and other distortions by carefully combining lenses made of different glasses.
The Troublesome Rainbow
In the 1660s, Isaac Newton, the great 17th-century scientist, observed that a glass prism spreads white light into a spectrum of colors. The effect, called dispersion, is beautiful—but it interferes with an image seen through the prism. In a lens, dispersion causes chromatic aberration, a fringe of color that blurs the image.
Newton believed that the chromatic aberration inherent to a glass lens couldn’t be corrected. With the single type of optical glass available to him. Newton didn’t see a solution to the problem, and his authority was such that he continued to search for an answer. But in 1733, a partial solution was discovered—by an amateur astronomer.
Chester Moor Hall had heard that lead-containing flint glass—discovered a few years after Newton had given up on lenses—had a greater ability to disperse light than ordinary crown glass. He ordered a concave lens made from flint glass, and a convex lens made from crown glass. When he stacked the two lenses, they partly canceled each other’s chromatic aberration. Although traces of color remained, Hall’s achromatic doublet was a dramatic improvement.
Correcting the image
- The convex lens focuses each color, or wavelength, of light at a different point. At high magnifications, the effect appears as a multicolored fringe around an image seen through a lens.
- Adding a concave lens, made from glass with a greater ability to disperse light, focuses the red and blue light to the same point. The fringe partly disappears.
- When another convex lens is added, the green light comes into the same focus as the red and blue light. Even more of the fringe is eliminated.
Published on October 25, 2011