Object distance and image distance – Convex mirrors

The object distance is the distance from the object to the mirror. This is denoted by the symbol $do$ .

Image distance (sometimes confused with the focal length) is the distance from the virtual image to the mirror. This is denoted by the symbol $di$ .

Ray diagram showing the object distance and image distance of a convex mirror

With the principal focal length added this would look like:

Ray diagram showing principal focal distance of a convex mirror