General rules for making ray diagrams
When an object is placed in front of a lens, the object's light rays strike the lens. The position, size, and nature of the formed image can only be understood if certain basic rules are followed.
When a ray of light strikes the convex or concave lens obliquely at its optical centre, it continues to follow its path without any deviation.
When rays parallel to the principal axis strikes a convex or concave lens, the refracted rays are converged to (convex lens) or appear to diverge from (concave lens) the principal focus.
When a ray passing through (convex lens) or directed towards (concave lens), the principal focus strikes a convex or concave lens, the refracted ray will be parallel to the principal axis.