When light waves travel from rarer medium to denser medium, the...

When light travels from one medium to another, the frequency of the light remains constant as it is a characteristic property of the source of light.

The velocity of light changes depending on the refractive index of the medium, given by $v = \dfrac{c}{\mu}$.

Since $v = \nu \lambda$ and the frequency $\nu$ is constant, the wavelength $\lambda$ must also change proportionally with the velocity.

Thus, velocity and wavelength change when light travels from a rarer medium to a denser medium.

Answer: velocity and wavelength.