The resolution of a telescope where objective has a diameter of 200 inches when the light of wavelength 4000 Å is coming from the star is
Answer & explanation
Correct answer: option 1
The correct answer is Option (1) → \( 9.606 \times 10^{-8}\,\mathrm{rad} \)
The limit of resolution of a telescope is the smallest angular separation it can resolve. Because of diffraction at the circular objective of diameter \( D \), it is
\( \displaystyle \Delta\theta = \frac{1.22\,\lambda}{D} \)
\( D = 200\,\mathrm{inch} = 200 \times 2.54\,\mathrm{cm} = 5.08\,\mathrm{m} \)
\( \lambda = 4000\,\mathring{\mathrm{A}} = 4\times10^{-7}\,\mathrm{m} \)
\( \displaystyle \Delta\theta = \frac{1.22 \times 4\times10^{-7}}{5.08} = \frac{4.88\times10^{-7}}{5.08}\,\mathrm{rad} \)
\( \Delta\theta = 9.606\times10^{-8}\,\mathrm{rad} \)
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 – …
