Optics Question 440
Question: A compound microscope has an eye piece of focal length 10 cm and an objective of focal length 4 cm. Calculate the magnification, if an object is kept at a distance of 5 cm from the objective so that final image is formed at the least distance vision (20 cm)
[UP SEAT 2005]
Options:
A) 12
B) 11
C) 10
D) 13
Show Answer
Answer:
Correct Answer: A
Solution:
For objective lens $ \frac{1}{f _{o}}=\frac{1}{v _{o}}-\frac{1}{u _{o}} $
Therefore $ \frac{1}{v _{o}}=\frac{1}{f _{o}}+\frac{1}{u _{o}} $
$ =\frac{1}{4}+\frac{1}{-5}=\frac{1}{20} $
Therefore $ v _{o}=20cm $ Now $ M=\frac{v _{o}}{u _{o}}( 1+\frac{D}{f _{e}} ) $
$ =\frac{20}{5}( 1+\frac{20}{10} ) $
$ =12. $
