Ray Optics and Optical Instruments - Result Question 71
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75. An astronomical telescope has a length of 44 $cm$ and tenfold magnification. The focal length of the objective lens is
======= ####75. An astronomical telescope has a length of 44 $cm$ and tenfold magnification. The focal length of the objective lens is
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(b) $40 cm$
(c) $44 cm$
(d) $440 cm$
[1997]
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Answer:
Correct Answer: 75. (b)
Solution:
- (b) Given : Length of astronomical telescope
$(f_0+f_e)=44 cm$ and magnification $(\frac{f_0}{f_e})=10$.
From the given magnification, we find that $f_0=10 f_e$. Therefore, $10 f_e+f_e=44$ or $11 f_e=44$ or $f_e$ $=4$. And focal length of the objective $(f_0)=44-f_e=44-4=40 cm$.
In case if objective and eye lens of a telescope are interchanged, it will not behave as a microscope but object appears very small.
