A person suffering from presbyopia needs bifocal lens. If he needs two lens of power - 4.0 dioptre and + 2.0 dioptre, which one of these two lenses is for the correction of distant vision and what is its focal length?

Presbyopia is an age-related defect where the power of accommodation decreases. When a person suffers from both myopia (inability to see distant objects clearly) and hypermetropia (inability to see near objects clearly), they require bifocal lenses. The upper part of a bifocal lens is a concave lens (negative power) to assist with distant vision, while the lower part is a convex lens (positive power) to assist with reading or near vision. Thus, the $-4.0\text{ D}$ lens is concave and meant for distant vision, whereas the $+2.0\text{ D}$ lens is convex and meant for near vision.