The speed of light decreases when light moves into a medium with a higher index of refraction (denser medium). It has been observed that the decrease in speed causes a change in the direction of the light ray. Note that light can also undergo a reflection from the surface.

O Ray O Wave Material Air 30 30 Index of Refraction (n) 1.00 50 Air Water Glass 90 90 60 Material Custom 60 Index of Refraction (n) 1.54 30 Air Water Glassnormal to surface incident ray reflected ray refracted ray @ 2006 Encyclopedia Britannica, Inc