Ray Optics And Optical Instruments Ques 73
73. An astronomical telescope has a length of $44 $ $cm$ and tenfold magnification. The focal length of the objective lens is
[1997]
(a) $4 $ $cm$
(b) $40 $ $cm$
(c) $44 $ $cm$
(d) $440 $ $cm$
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Answer:
Correct Answer: 73.(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.
BETA
