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Ray Optics — JEE physics STD 12 Science — Question

CBSE BoardEnglish MediumSTD 12 ScienceJEE physicsRay Optics1 Mark
MCQ
An astronomical telescope has an angular magnification of magnitude $5$ for distant objects. The separation between the objective and the eye piece is $36 \mathrm{~cm}$ and the final image is formed at infinity. The focal length $f$ of the objective and the focal length $f$ of the eye piece are
  • A
    $f=45 \mathrm{~cm}$ and $f=-9 \mathrm{~cm}$
  • B
    $f=7.2 \mathrm{~cm}$ and $f=5 \mathrm{~cm}$
  • C
    $f=50 \mathrm{~cm}$ and $f=10 \mathrm{~cm}$
  • ✓
    $f=30 \mathrm{~cm}$ and $f=6 \mathrm{~cm}$
✓

Answer

Correct option: D.
$f=30 \mathrm{~cm}$ and $f=6 \mathrm{~cm}$
In this case $|m|=\frac{f_o}{f_e}=5$and length of telescope $=f_o+f_e=36$
Solving (i) and (ii), we get $f=6 \mathrm{~cm}, f_o=30 \mathrm{~cm}$.

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