Distance of ray optics as a good approximation, when the aperture is 5 mm wide and the wavelength is 600 nm is nearly :
Answer & explanation
Correct answer: option 1
The correct answer is Option (1) → 41.67 m
Ray optics is a good approximation up to about the Fresnel distance \( z_F \). Beyond it the spreading of the beam due to diffraction becomes comparable to the size of the aperture.
\( \displaystyle z_F = \frac{a^2}{\lambda} \)
\( a = 5\,\mathrm{mm} = 5\times10^{-3}\,\mathrm{m} \)
\( \lambda = 600\,\mathrm{nm} = 6\times10^{-7}\,\mathrm{m} \)
\( \displaystyle z_F = \frac{(5\times10^{-3})^2}{6\times10^{-7}} = \frac{25\times10^{-6}}{6\times10^{-7}} = 41.67\,\mathrm{m} \)
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):
The Validity of Ray Optics – …
