The wavelength of light coming from a sodium source is 589 nm. Its wavelength in crown glass having refractive index 1.52 is
Answer & explanation
Correct answer: option 1
The correct answer is Option (1) → 387.5 nm
When light enters a medium, its frequency does not change, but its speed falls to \( v = c/n \). Since \( v = \nu\lambda \), the wavelength falls by the same factor \( n \).
\( \displaystyle \lambda_{\mathrm{glass}} = \frac{\lambda_{\mathrm{air}}}{n} \)
\( \displaystyle \lambda_{\mathrm{glass}} = \frac{589\,\mathrm{nm}}{1.52} = 387.5\,\mathrm{nm} \)
Note: NTA picked this question from an NCERT (2026-27) exercise and changed the figures. Do not ignore NCERT while practising numericals.
NCERT (2026-27):
Exercises – …
