CGPET 2015 Solved Paper

Given, power P = 3D
f=

1

P

=

1

3

m=

100

3

‌cm.
u=−25‌cm.
Applying lens formula, we can write

1

v

−

1

u

=

1

f

⇒

1

v

+

1

25

=

3

100

⇒

1

v

=

3

100

=

1

25

=

3−4

100

=

−1

100

v=−100‌cm
Now, again using lens formula we can write

1

v

=

1

u

=

1

f

As, focal length of a far sighted person decreases
⇒

1

−100

−

1

u

>

1

1.7

=

10

17

[∵f<1.7‌cm]
⇒

1

u

<−

1

100

−

10

17

=

−17−1000

1700

=

−1017

1700

⇒u>

1700

1000

=−1.7=−1m(nearest‌option)