Question

What are the 3 lines drawn in finding the images with a lens?

Answer 1

In the process of finding an image with a lens, three basic lines are usually drawn, the lines are as follows:
LINE ONE: This line represents the ray that leaves the object and travels towards the center of the lens. This line is usually undeflected and it travels through a straight line. 
LINE TWO: This line represents the ray which travels from the object parallel to the axis of the lens. The line is deflected in such a way that it passes through the focal point of the lens. 
LINE THREE: This line represents the ray that travels from the object and which passes through the focal point on the near side prior to getting to the lens. This line is deflected in such a way that it travels parallel to the axis. <span />