The speed of light decreases.
The speed of light in a medium is inversely proportional to the refractive index of the medium.
##v=frac{v_0}{n}## where
##v## is the speed of light in a particular medium.
##v_0=3##x##10^8##m/s is the speed of light in vacuum whose refractive index is 1.
##n## is the refractive index of the medium.
In water the speed of light is ##frac{v_0}{1.33}=2.26##x##10^8##m/s.
In glass the speed of light is ##frac{v_0}{1.52}=1.97##x##10^8##m/s.