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Rv. This article is about the paraxial approximation, not about the word ''paraxial''. Wikipedia is an encyclopedia, not a dictionary. Consult a dictionary for etymology of words. |
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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}}, 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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