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Clarified range of validity dependence on approximation -- cosine and sine behave differently. |
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[[File:Small angle compare error.svg|thumbnail|Plot showing the error associated with the paraxial approximation. N.B. In this plot the cosine is approximated by <math>1-\theta^2/2</math>.]]
In [[geometric optics]], the '''paraxial approximation''' is a [[small-angle approximation]] used in [[Gaussian optics]] and [[Ray tracing (physics)|ray tracing]] of light through an optical system (such as a [[lens (optics)|lens]]).<ref name=Greivenkamp>
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