NCERT Class XII Chapter
Ray Optics and Optical Instruments
Questions With Solutions
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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.
m e = ( 1 +
) =
or m e = 1 +
=
or m e = 6 and u e =
cm
Now magnification by objective lens
m =m 0 × m e or m 0 =
=
= 5
Alsom 0 =
⇒ m 0 =
= − 5 (Since real image is formed by objective.)
So , the relationv 0 - 5 u 0
Now lens formula for objective lens
−
=
or
−
=
=
or 5 u 0 = 7.5 or u 0 = - 1.5 cm
Alsov o = – 5 u o = 7.5 cm
So, required distance between objective and eyepiece
L =v 0 + | u e | = 7.5 +
= 11.67 cm
Let the final image is formed at least distance of distinct vision for eyepiece.
Now magnification by objective lens
m =
Also
So , the relation
Now lens formula for objective lens
Also
So, required distance between objective and eyepiece
L =
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