Imagine a marching band marching at a steady speed across a paved parking lot. When they reach the edge of the parking lot they keep going into a grassy field, but they go more slowly in the grass than across the parking lot.

If the lines of the marching band are parallel to the interface the lines get closer together when the band crosses over to the grass...the decrease in speed causes a decrease in wavelength.

If the lines of the marching band are not parallel to the interface there is not only a decrease in wavelength but also a change in direction of the wave.