To increase magnification power of refracting type Telescope, we should increase:
Answer & explanation
Correct answer: option 1
The correct answer is Option (1) → the focal length of the objective
The magnifying power of a refracting telescope (final image at infinity) is the ratio of the focal lengths of the objective and the eyepiece:
\( \displaystyle m = \frac{f_o}{f_e} \)
So the magnifying power increases when the focal length of the objective increases or the focal length of the eyepiece decreases.
Option (1) the focal length of the objective — Correct. \( f_o \) is in the numerator, so a longer objective focal length gives a larger \( m \).
Option (2) the focal length of the eyepiece — Incorrect. \( f_e \) is in the denominator; a longer eyepiece focal length reduces \( m \).
Option (3) aperture of the objective — Incorrect. A larger objective aperture gathers more light and improves the resolving power, but the aperture does not appear in \( m \).
Option (4) aperture of the eyepiece — Incorrect. The aperture of the eyepiece does not appear in \( m \) either.
NCERT (2026-27):
Telescope – “… But here, the objective has a large focal length and a much larger aperture than the eyepiece. … The magnifying power m is the ratio of the angle \( \beta \) subtended at the eye by the final image to the angle \( \alpha \) which the object subtends at the lens or the eye. Hence
\( \displaystyle m \approx \frac{\beta}{\alpha} \approx \frac{h}{f_e}\cdot\frac{f_o}{h} = \frac{f_o}{f_e} \qquad (9.46) \)”