- Paraxial rays: The lens equation and other widely encountered optical expressions constitute approximations that are most acceptable for light beams closer to the principal axis, for which the approximations is valid. Such rays are known as paraxial rays.
- A paraxial ray is a ray that establishes a small angle towards the optical axis of the system as well as remains close to it throughout the entire system.
- The primary distinction between paraxial rays as well as principal axes seems to be the distance between themselves as well as the principal axis.
- The closer the incident rays are parallel to the principal axis (paraxial), the more concurrent are the reflected rays as the focus is only meaningful if the incident rays are close to the principal axis (paraxial).
Therefore, the paraxial rays are nothing but a set of incident rays on the mirrors which lie very close to the principal axis.