Page "Total internal reflection" Paragraph 55

One can demonstrate total internal reflection by filling a sink or bath with water, taking a glass tumbler, and placing it upside-down over the plug hole ( with the tumbler completely filled with water ).

While water remains both in the upturned tumbler and in the sink surrounding it, the plug hole and plug are visible since the angle of refraction between glass and water is not greater than the critical angle.

If the drain is opened and the tumbler is kept in position over the hole, the water in the tumbler drains out leaving the glass filled with air, and this then acts as the plug.

Viewing this from above, the tumbler now appears mirrored because light reflects off the air / glass interface.