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CBSE Class 12 Physics 2020 Delhi Set 1 Paper

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Question : 32 of 37
Marks: +1, -0
An optical instrument uses a lens of power 100D100 D for objective lens and 50D50 D for its eyepiece. When the tube length is kept at 25cm25 \text{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 =PO=100D=P_O=100 D
∴  fO=1100m=1cm\therefore \; f_O = \frac{1}{100} \text{m} = 1 \text{cm}
∴  Power of eyepiece=PE=50D\therefore \; \text{Power of eyepiece} = P_E = 50 D
∴  fE=150m=2cm\therefore \; f_E = \frac{1}{50} \text{m} = 2 \text{cm}
Length of tube =L=25cm= L = 25 \text{cm}
D=D= least distance of distinct vision =25cm=25 \text{cm}
Magnification =LfO×DfE= \frac{L}{f_O} \times \frac{D}{f_E}
∴  \therefore \; Magnification =251×252=312.5= \frac{25}{1} \times \frac{25}{2} = 312.5
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