Light rays leaving an object interact with a spherical mirror in such a way that an image of the object is formed. The rays either pass through the image, in which case the image is said to be real, or the rays appear to diverge away from the image on the opposite side of the mirror from where the object is, in which case the image is virtual.
If the mirror's focal length, f, and the distance of the object from the mirror, do, are both known, then the distance of the image from the mirror, di, can be found by solving the mirror equation:
1/f = 1/do + 1/di