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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
where  may be any of i,
 ,
u,
 , or  . This regime is known as first-order, paraxial, or
Gaussian optics.
 Gaussian Optics, Optics, Paraxial Refraction
© 1996-2007 Eric W. Weisstein
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