The paraxial approximation is an approximation to the full equations of optics that is valid in the limit of small angles from the optical axis. In the above figure (Schroeder 1999, p. 6, Fig. 2.1), primes represent image angles and distances, and unprimed represent the object, with n the index of refraction to the left of the y-axis and the index of refraction to the right of the y-axis. The paraxial approximation then assumes that
Gaussian Optics, Optics, Paraxial Refraction
References
Hecht, E. Optics, 3rd ed. Reading, MA: Addison-Wesley, 1998.
Schroeder, D. J. Astronomical Optics, 2nd ed. San Diego, CA: Academic Press, 1999.
