The method of drawing ray diagrams for double convex lens is described below. The description is applied to the task of drawing a ray diagram for an object located beyond the 2F point of a double convex lens. Pick a point on the top of the object and draw three incident rays traveling towards the lens. Using a straight edge, accurately draw one ray so that it passes exactly through the focal point on the way to the lens. Draw the second ray such that it travels exactly parallel to the principal axis.
Place arrowheads upon the rays to indicate their direction of travel.
Draw the third incident ray such that it travels directly to the exact center of the lens. Once these incident rays strike the lens, refract them according to the three rules of refraction for converging lenses. The ray that passes through the focal point on the way to the lens will refract and travel parallel to the principal axis. Use a straight edge to accurately draw its path. The ray which traveled parallel to the principal axis on the way to the lens will refract and travel through the focal point.