Resolving power of a telescope can be increased by increasing:

The correct answer is Option (4) → Diameter of Objective lens

The resolving power of a telescope is the reciprocal of the smallest angle between two distant objects that it can still show as separate. Diffraction at the objective sets this limit:

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

where \( D \) is the diameter of the objective and \( \lambda \) the wavelength.

Option (1) Wavelength of light incident at telescope — Incorrect. The resolving power is inversely proportional to the wavelength, so a longer wavelength lowers it.

Option (2) Diameter of eye piece — Incorrect. The eyepiece only magnifies the image formed by the objective; it cannot add detail that the objective has not resolved.

Option (3) Focal length of eyepiece — Incorrect. The focal length of the eyepiece changes the magnifying power, not the resolving power.

Option (4) Diameter of Objective lens — Correct. A wider objective collects light over a larger aperture and reduces the diffraction spread, so the resolving power increases.

Note: A similar question was also asked in CUET (UG) 2023 on 05-06-2023 (Slot 3).

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.”