A star is seen using a telescope whose objective lens has a diameter of 250 cm. The wavelength of light coming from the star is 500 nm. The limit of resolution of telescope is
Answer & explanation
Correct answer: option 2
The correct answer is Option (2) → \( 2.4 \times 10^{-7} \) radians
The limit of resolution of a telescope is the smallest angular separation it can resolve; it is set by diffraction at the objective of diameter \( D \).
\( \displaystyle \Delta\theta = \frac{1.22\,\lambda}{D} \)
\( \lambda = 500\,\mathrm{nm} = 5\times10^{-7}\,\mathrm{m} \)
\( D = 250\,\mathrm{cm} = 2.5\,\mathrm{m} \)
\( \displaystyle \Delta\theta = \frac{1.22 \times 5\times10^{-7}\,\mathrm{m}}{2.5\,\mathrm{m}} = 2.44\times10^{-7}\,\mathrm{rad} \approx 2.4\times10^{-7}\,\mathrm{rad} \)
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 – …
