Paraxial approximation

[1][2] A paraxial ray is a ray that makes a small angle (θ) to the optical axis of the system, and lies close to the axis throughout the system.

[1] Generally, this allows three important approximations (for θ in radians) for calculation of the ray's path, namely:[1] The paraxial approximation is used in Gaussian optics and first-order ray tracing.

[1] Ray transfer matrix analysis is one method that uses the approximation.

In some cases, the second-order approximation is also called "paraxial".

In dimensional analysis on optics equations radians are dimensionless and therefore can be ignored.