In optics, a conjugate plane or conjugate focal plane of a given plane P, is the plane P′ such that points on P are imaged on P′. If an object is moved to the point occupied by its image, then the moved object's new image will appear at the point where the object originated. In other words, the object and its image are interchangeable. This comes from the principle of reversibility which states light rays will travel along the originating path if the light's direction is reversed. Depending on how an optical system is designed, there can be multiple planes that are conjugate to a specific plane. The points that span conjugate planes are called conjugate points.