Your grandfather has enjoyed good vision for most of his 80 years. You observe him having trouble reading websites on his new iPad, squinting while he moves it towards and away from his face. You realize he might benefit from reading glasses. Since you were going out anyway, you offer to pick up a pair of glasses for him at the drug store. Consider that your grandfather is at his most comfortable reading posture when he is holding the iPad at a distance of 18 inches. a. What dioptric power of reading glasses should you get for your grandfather? Note that reading glasses are normally available in increments of 0.25 Diopter.

b. It occurs to you that you could have just shown grandpa how to make the displayed content appear larger on the iPad by adjusting the display "zoom". What percentage zoom (100% is the default view, 200% would make text and images twice as large) would give your grandfather a similar viewing experience to wearing the reading glasses you were planning to purchase?