A beam of 12cm microwaves reflects internally on the hypotenuse of a right angle paraffin prism. Bringing the hypotenuse of a second identical paraffin prism within a few cm of the first prism allows the wave to be transmitted straight through the two prisms.
The microwave sensing antenna can be located at the position of the internally reflected ray to show that this ray disappears when the second prism is moved close. Alternatively, the antenna can be located in the straight through direction to show that waves continue in a straight line when frustrated total internal reflection occurs, as shown in the photograph at the right above.
Note that with a single prism the evanescent wave extends a few wavelengths beyond the surface of the prism. The intensity of the microwaves decreases exponentially, so they are easily observed up to about 50cm from the exit surface of the prism.