upward. The sign convention for angles is chosen so that all of the angles shown
in Fig. 2.1 are positive. Slope angles u and u' are positive when a counter-
clockwise rotation of the corresponding ray about B or B' brings the ray into
coincidence with the z-axis. The angles of incidence and refraction, / and i\ are
positive when a clockwise rotation of
the
corresponding ray about point P brings
the ray into coincidence with the line PC. All rotations are made through acute
angles.
The advantage of these conventions for distances and angles is that both
refracting and reflecting surfaces can be treate
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