Without his contact lenses,Mr.Chong can focus from 0.80 m to infinity.What refractive power of the lenses does he require for normal reading (0.25 m from the eyes)?
A)1.25 diopters
B)2.75 diopters
C)4.00 diopters
D)5.25 diopters
E)-5.25 diopters

Question

Quizplus · Accepted Answer

The refractive power $P$ needed for a lens can be calculated using the lens formula $P = 1/f$, where $f$ is the focal length in meters. Mr. Chong can naturally focus from 0.80 m to infinity, meaning without correction, his near point is 0.80 m. For normal reading at 0.25 m, he needs a lens that changes his near point to 0.25 m. The total power needed is the difference between the powers required to focus at 0.25 m and to focus at 0.80 m naturally. Thus, $P = 1/0.25 - 1/0.80 = 4 - 1.25 = 2.75$ diopters.

Without His Contact Lenses,Mr