CBSE Class 12 Physics 2020 Delhi Set 1 Paper

Question : 32 of 37

Marks: +1, -0

An optical instrument uses a lens of power

100 {\D}

for objective lens and

50 {\D}

for its eyepiece. When the tube length is kept at

25 {\cm}

, the final image is formed at infinity.
(a) Identify the optical instrument.
(b) Calculate the magnification produced by the instrument.

Solution:

(a) The instrument is called compound microscope.
(b) Power of objective

=P_O=100 {\D}

∴ \;f_{ {\O}}=\;{1}/{100} {\m}=1 {\cm}

∴ \;\;\text " Power of eyepiece "\;= {\P}_{ {\E}}=50 {\D}

∴ \;f_{ {\E}}=\;{1}/{50} {\m}=2 {\cm}

Length of tube

= {\L}=25 {\cm}

{\D}=

least distance of distinct vision

=25 {\cm}

Magnification

=\;{ {\L}}/{f_O} × \;{ {\D}}/{f_E}

∴ \;\;

Magnification

=\;{25}/{1} × \;{25}/{2}=312.5

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