JEE Main 2022 · 27 June · evening · Single correct · +4 / −1
If a wave gets refracted into a denser medium, then which of the following is true?
- (a) wavelength, speed and frequency decreases.
- (b) wavelength increases, sped decreases and frequency remains constant.
- (c) wavelength and speed decreases but frequency remains constant.
- (d) wavelength, speed and frequency increases.
Answer and solution
Answer (c) wavelength and speed decreases but frequency remains constant.
Solution
Physics sir · 4 steps
Frequency is set by the source, so it never changes; only speed and wavelength adjust at the boundary.
- Frequency stays fixed. The frequency of a wave is decided by its source. At the boundary, the waves arriving per second must equal the waves leaving per second, so f is the same in both media.
- Speed drops. A denser medium has a larger refractive index n, and v = c/n. So the wave slows down after entering it.
λ = v/f
- Wavelength follows. With f fixed, λ is proportional to v. Since the speed decreases, the wavelength decreases by the same factor.
- So. Wavelength and speed both decrease while frequency stays constant, which is option (c).
The idea to keep. In refraction, frequency is the one quantity that never changes. Speed changes with the medium, and wavelength changes along with speed because λ = v/f.
Answer from the NTA final answer key. Solution by PrepWiser.