Question

A normal near point is 25 cm away. Bob is very farsighted with a near point at 100 cm. What is the power (in diopters) of the eyeglasses that Bob needs for reading?

Answer 1

let F be the focal length of required lens .

since bob wants to make the image of object at 25 cm at 100 cm to view it clearly , so we have

object distance =u=25 cm

image distance =v=100cm

Now according to lens formula

1/(focal lenght ) = 1/(image distance ) - 1/(object distance ) = 1/(100) - 1/(25 ) = -3 /100 cm -1 = -3 m-1

and also power=1/(focal lenght )=-3 m-1 = -3 diopters

so the power (in diopters) of the eyeglasses that Bob needs for reading is -3 Diopter