Final answer:
Chromatic aberration is caused by lenses refracting different wavelengths of light to differing extents, resulting in various focal points for each color. This leads to image blurring and color fringing, which can be partially corrected with multiple-lens systems such as achromatic doublets.
Step-by-step explanation:
Chromatic aberration occurs when light passes through a lens and the lens's index of refraction varies with color or wavelength of the light. This causes different colors to focus at different points, resulting in a blurring of the image and often colorful fringes around the edges. The index of refraction is typically higher for shorter wavelengths (like violet) than for longer wavelengths (like red), leading to more bending of the violet light and thus a closer focal point. Multiple-lens systems, such as achromatic doublets, partially correct chromatic aberrations by combining lenses made of different materials with varying indices of refraction to reduce this dispersion.
For instance, an achromatic doublet consists of a converging lens made of crown glass in contact with a diverging lens made of flint glass, which counteracts the chromatic aberration and reduces color fringing. Unfortunately, while this improves image quality, it also adds to the complexity and expense of optical systems such as cameras and telescopes.