Resolving power of a telescope can be increased by increasing:

The correct answer is Option (2) → the diameter of the objective

Because of diffraction at the objective, two distant stars are just resolved when their angular separation equals the limit of resolution \( \Delta\theta = \frac{1.22\lambda}{D} \), where D is the diameter of the objective. The resolving power is the reciprocal of this:

    \( \displaystyle \text{resolving power} = \frac{1}{\Delta\theta} = \frac{D}{1.22\,\lambda} \)

It grows with D and falls as \( \lambda \) grows. Only the objective and the wavelength appear in it.

Option (1) the wavelength — Incorrect. A larger \( \lambda \) makes \( \Delta\theta \) larger and the resolving power smaller.

Option (3) the diameter of the eye piece — Incorrect. The eyepiece only magnifies the image formed by the objective; the diffraction limit is set at the objective.

Option (4) the focal length of eye piece — Incorrect. This changes the magnifying power \( f_0/f_e \), not the resolving power.

Note: A similar question was also asked in CUET (UG) 2023 on 30-05-2023 (Slot 1).

NCERT (2026-27):
Telescope – “… The resolving power, or the ability to observe two objects distinctly, which are in very nearly the same direction, also depends on the diameter of the objective. So, the desirable aim in optical telescopes is to make them with objective of large diameter.”