Why is light refracted when it passes from air to water?

Because the speed and the wavelenth of the light changes

Explanation:

When light moves from a medium to a different medium, its speed changes. In particular, the speed of light in a certain medium is defined by the refractive index of the medium:

where

c is the speed of light in vacuum

v is the speed of light in the medium

So, the higher the refractive index, the slower the light into that medium.

Moreover, the wavelength of light is related to its speed, according to

where f is the frequency. The frequency of the light does not change when it passes from one medium to another, so the wavelength must change as well.

In this situation, we have light passing from air to water. Air has a refractive index of approximately 1.00, while water has a refractive index of approximately 1.33. From the definitions above, we can conclude that light slows down when passing from air to water, and so its wavelength decreases as well.

Moreover, the change in speed of the wave also causes the change in its direcion, according to Snell's law:

where n1 and n2 are the refractive index of the first and second medium, while are the angle between the direction of the light and the normal to the interface in the first and second medium respectively. This phenomenon is called refraction, and it tells us that when light moves between two mediums with different refractive index, it changes direction.