When light travels from a less optically dense medium

[Byrne]

When light travels from a less optically dense medium to a more optically dense medium, the refracted ray bends toward the normal (i.e. air -> water). The reverse is true when traveling from a less optically dense medium to a more optically dense medium.

Now... why is this? Why does a decrease in wave velocity cause the ray to bend toward the normal and not away?

Thanks.

 

[Doc Al]

One way to understand refraction is via Huygens' principle. Here's a useful animation: http://www.walter-fendt.de/ph11e/huygenspr.htm