A doctor prescribes a corrective lens with power +2.5 D. What type of lens is this, and what is its focal length?
Correct: B
A positive power (+2.5 D) indicates a converging lens, which is a convex lens. The focal length (f) is the reciprocal of the power (P) in meters: f = 1/P. So, f = 1 / (+2.5) m = 0.4 m. Converting to centimeters, f = 0.4 * 100 cm = +40 cm. The positive focal length also confirms it is a convex lens.