Question

A far sighted person has a near point of 1.5meters. The power of specacles required so that he can read a book at a distance of 25cm is

Answer 1

Given,

Near point = 1.5 m

Book distance = 25 cm = 0.25m

To find,

The power of the required spectacles.

Solution,

We can simply solve this mathematical problem by using the following mathematical process.

Here,

v = -1.5 m

u = -0.25m

According to the lens formula,

1/f = 1/v - 1/u

1/f = -1/1.5 + 1/0.25

1/f = 1/0.25 - 1/1.5

1/f = 100/25 - 10/15

1/f = 300-50/75

1/f = 250/75

1/f = 3.33

f = 1/3.33 m

Power of the spectacles (p) = 1/f = 1 ÷ 1/3.33 = +3.33 D

Hence, the power will be +3.33D