A paraxial beam of light rays, a beam of rays propagating along the axis of a centred optical system and forming very small angles with the axis and with the normals to the refracting and reflecting surfaces of the system. The basic relations that describe the formation of optical images in axially symmetric systems hold strictly only for a paraxial beam of rays. Only images produced by such rays are free of the aberrations of optical systems (apart from chromatic aberration in lens systems). In practice, however, a paraxial beam of rays is usually understood to mean a beam of rays passing at finite angles (a few degrees) for which the departures from the strict relations are so small that they can be neglected. The region around the optical axis of a system in which the rays can be regarded as paraxial is also called paraxial.