If a wave gets refracted into a denser medium, which of the following is true?
Answer & explanation
Correct answer: option 4
The correct answer is Option (4) → frequency remains the same but both wavelength and speed decrease
On refraction the frequency of a wave is fixed by the source and does not change. The speed in a medium of refractive index \( n \) is \( v = c/n \), and the wavelength is \( \lambda = v/\nu \).
Entering a denser medium means a larger \( n \), so
\( \displaystyle v = \frac{c}{n} \ \text{decreases}, \qquad \lambda = \frac{v}{\nu} \ \text{decreases (same } \nu\text{)} \)
Option (1) frequency, wavelength and speed increase — Incorrect. The frequency does not change, and the speed and wavelength fall.
Option (2) frequency, wavelength and speed decrease — Incorrect. The frequency stays the same.
Option (3) frequency remains constant but wavelength decreases and speed increases — Incorrect. The wavelength does decrease, but the speed decreases too.
Option (4) frequency remains the same but both wavelength and speed decrease — Correct. Same frequency, smaller speed and smaller wavelength.
NCERT (2026-27):
Refraction of a Plane Wave – “… The above equation implies that when a wave gets refracted into a denser medium (v1 > v2) the wavelength and the speed of propagation decrease but the frequency ν (= v/λ) remains the same.”