Question

a man with spectacles can read a book placed at 25cm. the lens used in the spectacles has power -2.0D. the man without spectacles can read the book at which distance?​

Answer 1

Answer:

v=-25cm p=-2.0D

1/f=p -2=1/f

f=1/-2=-0.5m

f= -500cm.

1/f=1/v-1/u

1/-500=1/-25-1/u

1/u=1/500-1/25

1/u=_1_-20_

500

u=500/19

= 26.3157cm

Answer 2

The object distance is 16.6 cm.

Explanation:

Given that,

Image distance = 25 cm

Power = -2.0 D

We need to calculate the focal length

Using formula of power

$f=\dfrac{100}{P}$

Put the value into the formula

$f=\dfrac{100}{2}$

$f=50\ cm$

We need to calculate the object distance

Using formula of lens

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

Put the value into the formula

$\dfrac{1}{-50}=\dfrac{1}{25}-\dfrac{1}{u}$

$-\dfrac{1}{u}=\dfrac{1}{-50}-\dfrac{1}{25}$

$u=16.6\ cm$

Hence, The object distance is 16.6 cm.

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Topic : optics

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