The resolving power of a refracting type telescope can be increased by :
Answer & explanation
Correct answer: option 1
The correct answer is Option (1) → Choosing a large diameter objective lens
The resolving power of a telescope is its ability to show two distant objects that are very close in angle as separate. Because of diffraction at the objective, the smallest angle it can resolve is
\( \displaystyle \Delta\theta = \frac{1.22\,\lambda}{D} \;\Rightarrow\; \text{resolving power} = \frac{1}{\Delta\theta} = \frac{D}{1.22\,\lambda} \)
where \( D \) is the diameter (aperture) of the objective and \( \lambda \) the wavelength. A wider objective makes the diffraction pattern of each point narrower, so the resolving power rises in proportion to \( D \).
Option (1) Choosing a large diameter objective lens — Correct. Resolving power is proportional to the aperture \( D \), which is why astronomical telescopes use large objectives.
Option (2) Choosing a small diameter objective lens — Incorrect. A smaller \( D \) spreads each diffraction pattern more, so close objects overlap and the resolving power falls.
Option (3) Increasing wavelength of light — Incorrect. Resolving power varies as \( 1/\lambda \), so a longer wavelength lowers it.
Option (4) Using an objective lens of large focal length — Incorrect. The focal length of the objective changes the magnifying power (\( f_o/f_e \)), not the resolving power, which does not depend on \( f_o \).
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.”