For an astronomical telescope having objective lens of focal length 10 m and eyepiece lens of focal length 10 cm, telescope's tube length and magnification respectively are _____________.
Fill in the blank with the correct answer from the options given below.
Answer & explanation
Correct answer: option 4
The correct answer is Option (4) → 1010 cm, 100
In normal adjustment an astronomical telescope forms the final image at infinity: the image formed by the objective lies at the focus of the eyepiece. So the tube length is the sum of the two focal lengths, and the magnifying power is their ratio.
\( f_o = 10\,\mathrm{m} = 1000\,\mathrm{cm} \)
\( f_e = 10\,\mathrm{cm} \)
\( L = f_o + f_e = 1000\,\mathrm{cm} + 10\,\mathrm{cm} = 1010\,\mathrm{cm} \)
\( \displaystyle m = \frac{f_o}{f_e} = \frac{1000\,\mathrm{cm}}{10\,\mathrm{cm}} = 100 \)
Note: NTA picked this question from an NCERT (2026-27) exercise and changed the figures. Do not ignore NCERT while practising numericals.
NCERT (2026-27):
Exercises – …
