Diameter of the objective lens of a telescope is 250 cm. For light of wavelength 600 nm, coming from a distant object, the limit of resolution of the telescope is close to:
Answer & explanation
Correct answer: option 3
The correct answer is Option (3) → \( 3\times10^{-7}\,\mathrm{rad} \)
The limit of resolution of a telescope, set by diffraction at the circular aperture of the objective, is the smallest angular separation it can resolve:
\( \displaystyle \Delta\theta = \frac{1.22\,\lambda}{D} \)
With \( \lambda = 600\,\mathrm{nm} = 6\times10^{-7}\,\mathrm{m} \) and \( D = 250\,\mathrm{cm} = 2.5\,\mathrm{m} \):
\( \displaystyle \Delta\theta = \frac{1.22\times6\times10^{-7}}{2.5}\,\mathrm{rad} = 2.93\times10^{-7}\,\mathrm{rad} \approx 3\times10^{-7}\,\mathrm{rad} \)
Note: This question is from the pre-rationalised NCERT book (2022-23). The topic was removed from the latest NCERT book (2026-27), and it also appears to be out of the CUET 2026 syllabus.
NCERT (Pre-rationalised edition, 2022-23):
Resolving Power of Optical Instruments – …
