what happens when light passes from air to water?

Answer 1

it refracts ( bends its course)
this is because of the change in density between the two substances

Answer 2

Its velocity is reduced and if it enters the water at any angle other than normal, the beam of light bends, i.e suffers refraction. The refractive index of water is about 1.3 and that means that the velocity of light in water is 1.3 times slower than that in air (which is almost same as that in vacuum).