Paraxial approximation

Summary

The paraxial approximation treats rays that stay close to the optical axis and make small angles with it. It replaces $\sin\theta$ and $\tan\theta$ by $\theta$ and $\cos\theta$ by $1$. Then the laws of reflection and refraction become linear, and so does the passage of rays through a whole optical system. It is the basis of first-order (Gaussian) optics: focal lengths, image positions and magnifications. See the definition.