When light travels from a less dense medium to a denser medium, its speed decreases. This is because the light waves interact more strongly with the atoms and molecules in the denser medium, which causes them to slow down.

As the light waves slow down, their wavelength decreases. This is because the wavelength of a light wave is inversely proportional to its speed. So, if the speed of a light wave decreases, its wavelength will increase.

The opposite is also true. When light travels from a denser medium to a less dense medium, its speed increases and its wavelength increases.

The change in wavelength of light when it enters a new medium is called refraction. Refraction is responsible for the bending of light when it passes from one medium to another.