Question

Why the speed of sound increase in water but that of light decreases ?

Answer 1

Because they are both different forms of waves first is the vibrational wave which propagates by vibrating the molecules of the medium the closer the molecules the more the vibration achieved and the better and faster the propagation.

Light wave is an Electromagnetic wave with a different mechanism of propagation altogether​ which involves absormtion and re-emission of the wave energy when it falls on the atoms of the medium, which causes the electrons to viibrate. These elctrons aftert he short vibrating period create a new electromagnetic wave having the same frequency as the absorbed wave. The time thus taken, though is very short but delays the motion of the wave through the medium and thus reduce the speed of the wave.