Optical Systems lecture 2

Lecture 2
1-1 Spherical aberration
Spherical aberrations occur because the focal length of a spherical lens or a mirror depends on the position of the monochromatic ray relative to the optical axis of the lens. In other words, spherical aberration is an optical effect observed in an optical device (lens, mirror, etc.) that occurs due to the increased refraction of light rays when they strike a lens or a reflection of light rays when they strike a mirror near its edge, in comparison with those that strike nearer the centre. Figure (1-1) shows an example of spherical aberration of converging lens with rays passing parallel to the optical axis of the lens.
A spherical lens has an aplanatic point (i.e., no spherical aberration) only at a radius that equals the radius of the sphere divided by the index of refraction of the lens material. A typical value of refractive index for crown glass is 1.5, which indicates that only about 43% of the area (67% of diameter) of a spherical lens is useful. It is often considered to be an imperfection of telescopes and other instruments which makes their focusing less than ideal due to the spherical shape of lenses and mirrors.