CGPET 2021 Solved Paper

According to the problem, focal length of objective, fo=50cm
Object distance, uo=−200cm
Image distance, vo=?
Now, from lens formula,

1

fo

=

1

vo

−

1

uo

⇒

1

vo

=

1

fo

+

1

uo

⇒

1

vo

=

1

50

+

1

(−200)

∴ vo=

200

3

cm........(i)
Also, focal length of eyepiece,
fe=5cm,ve=−D=−25cm
Again for eyepiece, using lens formula,

1

fe

=

1

ve

−

1

ue

⇒

1

ue

=

1

ve

−

1

fe

Put the given values, we get

1

ue

=

1

−25

−

1

5

∴ue=−

25

6

cm .....(ii)
Since, the separation between objective and eyepiece is
L=|vo|+|ue|
From eqs. (i) and (ii), we get
L=

200

3

+

25

6

⇒ L=70.83≃70.8cm