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Paraxial Approximation




A paraxial ray is a Ray which makes a small angle (''θ'') to the Optical Axis of the system, and lies close to the axis throughout the system. Generally, this allows three important approximations (for ''θ'' in Radian s) for calculation of the ray's path:

: \sin( heta) \approx heta
: an( heta) \approx heta
and
: \cos( heta) \approx 1

The paraxial approximation is used in ''first-order'' raytracing and Gaussian Optics . Ray Transfer Matrix Analysis is one method that uses the approximation.

In some cases, the second-order approximation is also called "paraxial". The approximations above for sine and tangent are already accurate to second order in ''θ'', but the approximation for cosine needs to be expanded by including the next term in the Taylor Series expansion. The third approximation then becomes

: \cos( heta) \approx 1 - { heta^2 \over 2 } \ .