IIT JEE Advanced 2006 Paper 1

From lens formula, we have

1

v

−

1

u

=

1

f

Here, f = 5 cm. Taking the least count (error) of the measuring scale to be 1 mm = 0.1 cm, for the worst case, we get

Δf

f

=

Δu

u

+

Δv

v

+

Δ(u+v)

u+v

(by using f =

uv

u+v

)
Therefore,

Δf

f

=

0.1

10

+

0.1

10

+

0.2

2.0

=

3

100

⇒ Δf =

3

100

×5 =

3

20

= 0.15
The closest answer is f = 5.00 ± 0.10 cm, that is, option (D).