You position an object in front of a spherical mirror for which the magnitude of the radius of curvature is \(R_{1}\). The image formed is real, inverted, and twice as large as the object. You then replace this spherical mirror with one that has a radius of curvature of magnitude \(R_{2}\) (keeping the object distance unchanged). The image formed by this mirror is virtual, upright, and twice as large as the object. Calculate a value for the ratio \(R_{2} / R_{1}\).