A short sighted person's range of vision is between 12 cm and 35 cm from the eye.What strength lens (in Diopters)is needed to move her near point to 25 cm?
A)-3.4 D
B)+3.4 D
C)-12.0 D
D)-4.3 D
E)+4.3 D

Question

Quizplus · Accepted Answer

The formula for calculating the power of a lens is P = 1/f, where P is the power in diopters and f is the focal distance in meters.
Near point = 12 cm = 0.12 m 
Far point = 35 cm = 0.35 m
We need to move the near point to 25 cm = 0.25 m
To find the power of the lens required, we need to find the difference between the current near point and the required near point, then take the reciprocal of that value. 
So, the power of the lens required is:
P = 1/ (0.25 - 0.12) = -4.3 D (negative sign indicates a concave lens needed for a myopic eye)

A Short Sighted Person's Range of Vision Is Between 12