We know the speed at which a wave travels is dependant upon the medium it travels through. A number called the refractive index of a substance is a measure of how much the speed of a wave changes compared to the speed in a reference medium i.e. air or a vacuum.
For light waves the refractive index is given as
The refractive index for glass is approximately 1.5; this means the speed of light in glass is approximately 1.5 times slower than the speed of light in air. Similarly, the refractive index of water is 1.33. This implies that light rays will not be bent as much in water as they are in glass as they are not slowed down as much in water.