To locate an image, any two of these three rays can be chosen. The images are of two types: real and virtual.
Real image of an object is formed when reflected rays actually intersect. These images are inverted and can be projected on a screen. They are formed on the same side as the
object in front of the mirror.
Virtual image of an object is formed by reflected rays that appear to diverge from the mirror. Such images are always erect and virual; these cannot be projected on a screen. They are formed behind the mirror.
The sign convention is followed based on the cartesian coordinate system. While using this convention, the following points should be kept in mind:
1. All distances are measured from the pole (O) of the mirror. The object is always placed on the left so that the incident ray is always taken as travelling from left to right.
2. All the distances on the left of O are taken as negative and those on the right of O as positive.
3. The distances measured above and normal to the principal axis are taken as positive and the downward distances as negative.
4. The radius of curvature and the focal length of a concave mirror are negative and those for a convex mirror are positive.