When an optician prescribes a -5D lens, it means that the lens is a concave lens with a focal length of 20 cm. The D in -5D represents the diopter, which is a unit of measurement for the optical power of a lens. The diopter indicates the degree of convergence or divergence of light that the lens can achieve.
Option 1 is the correct answer because a concave lens is used when a person has myopia or nearsightedness. In myopia, the light entering the eye focuses in front of the retina instead of directly on it. By prescribing a -5D lens, the optician is correcting the myopia by making the light rays diverge, allowing them to focus properly on the retina.
Options 2, 3, and 4 are incorrect because they mention either a convex lens or a concave lens with a different focal length. A convex lens is used to correct hyperopia or farsightedness, where the light focuses behind the retina. The mentioned focal lengths are also different from the 20 cm focal length prescribed in the question.
Overall, a -5D lens is a concave lens with a 20 cm focal length, used to correct myopia or nearsightedness