The focal length f and the radius of curvature R = 2f are positive for a
concave mirror and negative for a convex mirror. The perpendicular
distance of the object from the mirror surface is xo, and the
perpendicular distance of the image from this surface is xi.
Distances in front of the
mirror are positive and distances behind the mirror are negative negative.
In the paraxial approximation xo and xi satisfy the mirror equation, 1/xo + 1/xi = 1/f = 2/R. The magnification M is the ratio of the height of the image hi to the height of the object ho. M = hi/ho= -xi /xo. If the magnification is negative, the image is inverted.
We can determine the positions and sizes of images of points formed by spherical mirrors geometrically by drawing ray diagrams. Only two incident rays and their reflections must be drawn. The intersection of the two reflected rays, or, for divergent rays, the intersection of their backward extensions, marks the position of the image of your chosen point on the object. Choose two or more of the rays listed below.