What are paraxial rays?

(N/A) Paraxial rays are light rays that travel very close to the principal axis of an optical system (such as a spherical mirror or lens) and make very small angles with it.
Because these rays are close to the axis,the angle of incidence $\theta$ is small,allowing for the approximation $\sin \theta \approx \theta$ and $\tan \theta \approx \theta$.
This approximation is fundamental in deriving the mirror formula $\frac{1}{f} = \frac{1}{v} + \frac{1}{u}$ and the lens formula.