Ray diagrams are useful tools for determining the location of an image as produced by a concave mirror. Of all the rays which emanate from the top of the object arrow and are incident to the mirror, there are two rays whose behavior at the mirror surface can be easily predicted. These are the two incident rays which are used in the ray construction. All concave and convex mirror ray diagrams can be constructed from knowledge of the behavior of two rays. Learn more here about ray diagrams.