Which of the following happens when a ray of light enters into a water filled glass beaker from air?
Answer & explanation
Correct answer: option 3
The correct answer is Option (3) → wavelength of light decreases
The frequency of light is fixed by its source and does not change at a boundary. The speed in a medium of refractive index \( n \) is \( v = c/n \), which is smaller in water than in air, and \( v = \nu\lambda \) then requires a smaller wavelength.
\( \displaystyle \lambda_{\mathrm{water}} = \frac{v}{\nu} = \frac{c}{n\nu} = \frac{\lambda_{\mathrm{air}}}{n} \lt \lambda_{\mathrm{air}} \quad (n \approx 1.33) \)
So the wavelength decreases while the frequency stays the same, which makes Options (1), (2) and (4) wrong.
NCERT (2026-27):
Refraction of a Plane Wave – “… The above equation implies that when a wave gets refracted into a denser medium (\( v_1 \gt v_2 \)) the wavelength and the speed of propagation decrease but the frequency \( \nu \) (= \( v/\lambda \)) remains the same.”