If a wave gets refracted into a denser medium, then which of the following is true?

When a wave travels from a rarer medium to a denser medium (refraction into a denser medium), we need to determine what happens to its wavelength, speed, and frequency.

Frequency remains constant during refraction.

When a wave crosses a boundary between two media, the frequency does not change. This is because the number of wave crests arriving at the boundary per second must equal the number leaving — frequency is determined by the source.

Speed decreases in a denser medium.

In a denser medium, the speed of a wave decreases. For example, the speed of light in a medium is $$v = \frac{c}{n}$$, where $$n > 1$$ is the refractive index of the denser medium.

Wavelength decreases.

Since $$v = f\lambda$$ and frequency $$f$$ remains constant while speed $$v$$ decreases, the wavelength $$\lambda$$ must also decrease:

$$\lambda_{\text{denser}} = \frac{v_{\text{denser}}}{f} < \frac{v_{\text{rarer}}}{f} = \lambda_{\text{rarer}}$$

Therefore, wavelength and speed decrease but frequency remains constant.

The correct answer is Option C.