Question

An individual is nearsighted; his near point is 18.0 cm and his far point is 40.0 cm.

(a) What lens power is needed to correct his nearsightedness?
-2.5diopters
(b) When the lenses are in use, what is this person's near point?...?

Answer 1

given,

near point = 18 cm

far point = 40 cm

a) The lens should form an upright, virtual image at far point from the distant object.

therefore, f = q = -40 cm = -0.4 m

where f is the focal length.

the required power

`P =(1)/(f)`

`P =(1)/(-0.40)`

P = -2.5 D

b) If the lens is used the Person's near point

The lens should form an upright, virtual image at near point from the distant object should be q = - 18 cm = -0.18 m

`p = (qf)/(q-f)`

`p = ((-0.18)(-0.4))/(-0.18-(-0.4))`

p = 32.72 cm

The person's near point is 32.72 cm