Sometimes a person cannot clearly see objects close up or far away. To correct this type of vision, bifocals are often used. The top half of the lens is used to view distant objects and the bottom half of the lens is used to view objects close to the eye. A person can clearly see objects only if they are located between 42 cm and 162 cm away from her eyes. Bifocal lenses are used to correct her vision. What power lens (in diopters) should be used in the top half of the lens to allow her to clearly see distant objects?