Chromatic and Spherical Aberration

Real lenses and mirrors do not bring all rays to a single perfect focus. Two faults, called aberrations, blur the image.Chromatic aberration happens in lenses. The refractive index of glass depends on the wavelength of light, so different colours are refracted by different amounts. Blue light is refracted more than red light, so a converging lens brings blue light to a focus closer to the lens than red light. There is no single focal point, so the image is blurred and has coloured fringes.Chromatic aberration can be reduced by an achromatic doublet: a converging lens of crown glass cemented to a weaker diverging lens of flint glass, which cancels the spreading of colours. Mirrors reflect all wavelengths in the same way and do not refract light, so reflecting telescopes do not suffer from chromatic aberration.Spherical aberration happens with both lenses and mirrors whose surfaces are parts of spheres. Rays far from the principal axis are brought to a focus closer to the lens or mirror than rays near the axis, so again there is no single focus and the image is blurred.Spherical aberration is removed in reflecting telescopes by using a parabolic mirror, which focuses all rays parallel to the axis to one point. In lenses it can be reduced by using only the central part of the lens or by specially shaped surfaces.These aberrations are one reason large research telescopes are reflectors rather than refractors.