When a ray is traveling from air to water

1.) the ray will refract toward the normal. Light travels slower in water than in air, since water has a larger index of refraction (n = c/v). The part of the wavefront that reaches the air-water interface first will travel slower than the rest of the wavefront. This is what causes the light ray to bend toward the normal.

2.) Water is more dense than air. This is part of the reason why water has a higher index of refraction than air.

 

cold air is more dense than hot air, which is why your car engine runs more efficiently in colder weather...because the compression is higher.

But yeah, index of refraction is a function of temperature. This explains why you see mirages (puddles of water) on the road on a hot sunny day. The air is hotter near the road and gets colder as you go up. This temperature gradient refracts sunlight to the point where it appears as if it is coming from the road, causing the appearance of a mirage.

 

Hot air is less dense than cold air. Hot air refracts less, but the mixture of hot air and cold air is what creates the 'wavy' effect.

 

Interesting question. According to wikipedia:

"In general, the refractive index of a glass increases with its density. However, there does not exist an overall linear relation between the refractive index and the density for all silicate and borosilicate glasses. A relatively high refractive index and low density can be obtained with glasses containing light metal oxides such as Li2O and MgO, while the opposite trend is observed with glasses containing PbO and BaO as seen in the diagram at the right."

 

Intead of 'density', use 'index of refraction': as DavidSnyder pointed out, it's possible to have a dense low refractive index glass.

Then, as a ray crosses a boundary between a low index material n1 and high index material n2, if the ray goes from n1 to n2 it will refract towards the normal, and vice-versa.