Ray 3 goes through the focal point and then travels parallel to the axis after passing through the lens. Thus any point on the object can be mapped, using the rays above, into a corresponding point on the image. This point is located on the intersection of the rays. The above is a useful tecnique, but it usually involves a drawing of some sort. A more practical way is not as complete but is much simpler.
Interesting things happen when d o is equal to or greater than f .
As the object is moved closer toward the lens, the image distance tends to zero from the negative side. This is called a virtual image represented by a gray arrow in this applet.