As light undergoes refraction into a medium,

When light travels from vacuum or air into an optically denser medium of refractive index $n$, its speed decreases according to the relation $v = \dfrac{c}{n}$.

The frequency $f$ of light is a fundamental property of the source and remains unchanged during refraction across any boundary.

Using the wave equation $v = f \lambda$, since the speed $v$ decreases and the frequency $f$ remains constant, the wavelength $\lambda$ must decrease ($\lambda' = \dfrac{\lambda}{n}$).

Therefore, its wavelength decreases while its frequency remains unchanged.