Chapter-9|Ray Optics and Optical Instrument|NCERT 12th Physics:
Image Formation in Lenses Using Ray Diagrams:
(I) Rays which are parallel to the principal Axis after refraction, will passes through principal focus in case of convex lens and will appear to be coming from principal focus in case of concave lens.
(II) Rays passing through or directed to the focus will emerge parallel to the principal axis.
(III) Rays directed to the optical centre will emerge out undiverted.