Question

a woman can read the large print in a newspaper only when it is at a distance of 70cm or more from her eyes.
calculate the refractive power of the lens required to correct this defect (assume that the corrective lenses are worn 2cm from her eyes)

Answer 1

Farsightedness, or hyperopia person can't see near object clearly

This is corrected by convex lens

So, we need the lens such that object appear to be at a distance of nearpoint which is 70.0 cm here.

Distance between object and lens, do = (distance of object - distance between eye and lens)

do = 23.0 cm

= 0.23 m

Distance between image and lens, di = -(nearpoint - distance between eye and lens)

Since image here is virtual image, sign of di is negative

di = -68.0 cm

= -0.68 m

1/f = 1/do + 1/di

1/f = 1/0.23 + 1/-0.68

1/f = 4.348 + -1.471

1/f = 2.877

f = 0.3476 m

Given:

f = 0.3476 m

Use:

P = 1/f

P = 1/0.3476

f = 2.877 D

Answer: 2.88 D