Lens aberration is the deviation of light rays from a single focus; a defect in a focusing mechanism that prevents the intended focal point.

Chromatic aberration is when blue light is refracted more than red light, so the colors focus differently. This is caused by the dependence of a lens’s index of refraction on color (wavelength). The lens is more powerful for violet (V) than for red (R), producing images with different locations and magnifications.

Multiple-lens systems, such as this achromatic doublet, can partially correct chromatic aberrations, but they may require lenses of different materials and add to the expense of optical systems such as cameras.

Key Points

• Lens aberration is the deviation of light rays from a single focus

• Chromatic aberration caused by the wavelength-dependent index of refraction in glass

• The use of multiple elements allows for the correction of more optical aberrations, such as the chromatic aberration.

Key Terms

achromatic doublet: A type of lens made up of two simple lenses paired together designed so that the chromatic aberration of each lens partially offsets the other; in this way, light in a range of wavelengths may be brought to the same focus

chromatic aberration: when colors are incorrectly refracted by the lens, resulting in a mismatch at the focal point where the colors do not combine as they should