Explanation:

The upper and lower portions in common type of bifocal lenses are concave and convex respectively .Bifocals are eyeglasses with two distinct optical powers. Bifocals are commonly prescribed to people with presbyopia who also require a correction for myopia, hyperopia, and/or astigmatism. Bifocal contacts lenses have two prescriptions in the same lens. Multifocal contact lenses have a range of powers (similar to progressive eyeglass lenses) in each lens. "Multifocal contacts" also is used as a catch-all term for all contact lenses with more than one power, including bifocal contacts.