- Incident light rays that form a small angle $\theta$ with the mirror axis, giving them special properties, like where they will (approximately) focus and image for certain geometries
- Paraxial rays in the Spherical Mirror are approximated by a Paraboloidal Mirror with a focus $-R/2$ from the center and vertex $-R/2$ from the focus (or $-R$ from the focus)
- Possible due to the small angle approximation
- Entire branch of optics dealing with paraxial rays, known as paraxial optics/first-order/Gaussian optics