Question

why does the speed if light increases when it enters from water to air

Answer 1

The speed of light, like any other electromagnetic phenomenon, is governed by Maxwell's equations. They say that the sped of light depends on the electric permittivity and magnetic permeability. The permittivity of air is more than that of water and that gives a good enough reason. As to why the permittivity is more, that depends on a lot of factors. Distance between molecules, number of free electrons and many others