Question

Mary's glasses have +4.4D converging lenses. This gives her a near point of 20cm. What is the location of her near point when she is not wearing her glasses?
Express your answer with the appropriate units.

Answer 1

Answer:

The position of near point is 1.68 m.

Explanation:

Given that,

Power = + 4.4 D

Object distance = 20 cm

We need to calculate the focal length

Using formula of power

$P =\dfrac{1}{f}$

$f=\dfrac{1}{P}$

$f=\dfrac{1}{4.4}$

$f=0.227\ m$

We need to calculate the image distance

Using formula of lens

$\dfrac{1}{f}=\dfrac{1}{v}-\dfrac{1}{u}$

$\dfrac{1}{v}=\dfrac{1}{f}+\dfrac{1}{u}$

Put the value into the formula

$\dfrac{1}{v}=\dfrac{1}{0.227}-\dfrac{1}{0.20}$

$\dfrac{1}{v}=-\dfrac{135}{227}$

$v=-1.68\ m$

Hence, The position of near point is 1.68 m.