Chapter 48
Optical Defects
A number of defects, oraberrationscan occur with mirrors and lenses that prevent them from forming an
ideal image. A few of these defects are described here.
48.1 Spherical Aberration
If the surface of a mirror deviates from its ideal paraboloidal shape, or the surface of a lens deviates from its
ideal hyperboloidal shape, (for example, if the optical surfaces are sections of spheres), then the mirror or
lens is said to have aspherical aberration. If a lens or mirror has a spherical aberration, then light rays far
from the axis focus at a different point than light rays near the axis, causing a blurring of the image. (See Fig.
48.1, top.)
48.2 Chromatic Aberration
In lenses, light of different wavelengths will generally focus at different points. This phenomenon (to be
described later) is calleddispersion, and is the variation of index of refraction with wavelength. This effect in
lenses causes a defect calledchromatic aberration, which causes the image to be surrounded by a rainbow-
like halo. It can be corrected by using combinations of several lenses, each made of a material of a different
index of refraction. Chromatic aberration does not occur in mirrors. (See Fig. 48.1, bottom.)
48.3 Astigmatism
Astigmatismis caused by an asymmetrical lens or mirror, and causes light along different axes to be focused
at different points.
48.4 Coma
Another type of optical aberration is calledcoma. Coma does not affect light rays parallel to the optical axis,
but light rays from objectsoff-axistend to be smeared into a comet-like shape.