Chromatic aberration

We have seen that both mirrors and lenses suffer from spherical aberration, an effect which limits the clarity and sharpness of the images formed by such devices. However, lenses also suffer from another type of abberation called chromatic abberation. This occurs because the index of refraction of the glass in a lens is different for different wavelengths. We have seen that a prism refracts violet light more than red light. The same is true of lenses. As a result, a simple lens focuses violet light closer to the lens than it focuses red light. Hence, white light produces a slightly blurred image of an object, with coloured edges.

For many years, chromatic abberation was a sufficiently serious problem for lenses that scientists tried to find ways of reducing the number of lenses in scientific instruments, or even eliminating them all together. For instance, Isaac Newton developed a type of telescope, now called the Newtonian telescope, which uses a mirror instead of a lens to collect light. However, in 1758, John Dollond, an English optician, discovered a way to eliminate chromatic abberation. He combined two lenses, one converging, the other diverging, to make an achromatic doublet. The two lenses in an achromatic doublet are made of different type of glass with indices of refraction chosen such that the combination brings any two chosen colours to the same sharp focus.

Modern scientific instruments use compound lenses (i.e., combinations of simple lenses) to simultaneously eliminate both chromatic and spherical aberration.