2.2.3 The Gaussian optics approximation
Figs. 2.1 and 2.2 represent refraction effects in a plane, the plane of the paper or principal section, as a two-dimensional process. The most important aspects of the theory of centered optical systems can be dealt with in this section: the y, z plane, where z is the direction of the optical (system) axis and y the height in this section above the axis. Any light ray not lying in this section, i.e. for which the x coordinate is not always zero, is called a skew ray. We shall refer to these in Chap. 3.
In the principal section, any surface of a centered optical system can be defined, with origin at its intersection with the axis, by the equation (see
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