Waves travel through the blue region at half the speed as they travel through the gray region. This demonstration shows short pulses hitting the boundary between the regions at an angle from inside the blue region. The angle is such that none of the main part of the wave is transmitted; this is called total internal reflection. You will see some activity in the gray area; this is partly because the top part of the wave is rounded instead of being a plane, so that it hits the interface at a different angle (it goes up from the source instead of diagonally). This part is transmitted, but the plane part going diagonally is reflected. But, even for the part of the wave that is reflected, you will a portion of the wave travelling along the interface between the blue and gray area; but it will not propagate into the gray area.

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