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\tan \theta &\approx \theta
\end{align}</math>
and
:<math>\cos \theta \approx 1</math>
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:<math> \cos \theta \approx 1 - { \theta^2 \over 2 } \ .</math>
The paraxial approximation is fairly accurate for angles under about 10°{{Citation needed|date=May 2011}}, but is inaccurate for larger angles.
For larger angles it is often necessary to distinguish between [[meridional ray]]s, which lie in a plane containing the [[optical axis]], and [[sagittal ray]]s, which do not.
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== External links ==
* [http://demonstrations.wolfram.com/ParaxialApproximationAndTheMirror/ Paraxial Approximation and the Mirror] by David Schurig, [[The Wolfram Demonstrations Project]].
[[Category:Geometrical optics]]
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