NCERT Class XII Chapter
Ray Optics and Optical Instruments
Questions With Solutions

© examsnet.com
Question : 33
Total: 38
An angular magnification (magnifying power) of 30 X is desired using an objective of focal length 1.25 cm and an eyepiece of focal length 5 cm. How will you set up the compound microscope?
Solution:  
Here we want the distance between given objective and eye lens for the required magnification of 30.
Let the final image is formed at least distance of distinct vision for eyepiece.
me = (1+
D
fe
)
=
D
ue
or me = 1 +
25
5
=
25
ue
or me = 6 and ue =
25
6
cm
Now magnification by objective lens
m = m0×me or m0 =
m
me
=
30
6
= 5
Also m0 =
v0
u0
m0 =
v0
u0
= 5 (Since real image is formed by objective.)
So , the relation v0 - 5 u0
Now lens formula for objective lens
1
v0
1
u0
=
1
f0
or
1
5u0
1
u0
=
1
1.25

1+5
5u0
=
1
1.25
or 5u0 = 7.5 or u0 = - 1.5 cm
Also vo = – 5 uo = 7.5 cm
So, required distance between objective and eyepiece
L = v0 + |ue| = 7.5 +
25
6
= 11.67 cm
© examsnet.com
Go to Question: