What will be the wavelength of light from a sodium source in water (refractive index \( 1.33 \)) which has a wavelength of light \( 589 \) nm in air?
Answer & explanation
Correct answer: option 3
The correct answer is Option (3) → \( \sim 443 \) nm
On refraction the frequency stays the same (it is fixed by the source), while the speed falls by the factor \( \mu \); hence the wavelength also falls by \( \mu \).
\( \displaystyle \lambda_{w} = \frac{v}{f} = \frac{c/\mu}{f} = \frac{\lambda_{\mathrm{air}}}{\mu} \)
\( \displaystyle \lambda_{w} = \frac{589\,\mathrm{nm}}{1.33} \approx 443\,\mathrm{nm} \)
Note: This question is from NCERT (2026-27). NTA sets numerical questions from NCERT too, so do not ignore NCERT while practising numericals.
NCERT (2026-27):
Exercises – …
