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The focal length of the objective of a terrestrial telescope is $80 \mathrm{~cm}$ and it is adjusted for parallel rays, then its magnifying power is 20 . If the focal length of erecting lens is $20 \mathrm{~cm}$, then full length of telescope will be
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$164 \mathrm{~cm}$
$m=\frac{f_o}{f_e} \Rightarrow \frac{80}{f_e}=20 \Rightarrow f_e=4 \mathrm{~cm}$
Hence length of terrestrial telescope
$=f_o+f_e+4 f=80+4+4 \times 20=164 \mathrm{~cm}$
Hence length of terrestrial telescope
$=f_o+f_e+4 f=80+4+4 \times 20=164 \mathrm{~cm}$
Hence length of terrestrial telescope
$=f_o+f_e+4 f=80+4+4 \times 20=164 \mathrm{~cm}$
Hence length of terrestrial telescope
$=f_o+f_e+4 f=80+4+4 \times 20=164 \mathrm{~cm}$
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