Final answer:
Concave mirrors and converging (convex) lenses both have a positive focal length following the sign convention in geometric optics. These devices focus light rays to a point and are used in applications like correcting farsighted vision.
Step-by-step explanation:
The two optical objects that have a positive focal length, based on the sign convention in geometric optics, are concave mirrors and converging lenses. A concave mirror, also known as a converging mirror, has a focal length (f) that is positive because it causes parallel rays to converge at a focal point on the same side as the object. Similarly, a converging lens, or convex lens, has a positive focal length since it brings rays together at a focal point on the opposite side of the lens from the object.
Furthermore, the sign convention in optics assigns a positive image distance (di) to real images and a negative one to virtual images, and a positive object distance (do) is typical when the object is in front of the mirror or lens.
As an example of how these rules apply, consider the convex spectacle lenses used for farsighted people; they have a positive focal length and a positive prescribed power, indicating their converging nature.