6C20.45 - Microwave Fresnel Zones
To demonstrate how to focus light using diffraction instead of refraction or reflection with microwaves.
The Fresnel Zone plate for microwaves consists of a wooden circle with concentric aluminum rings that sits on a wooden stand. A cover plate with the central hole cut out of it can be placed in front of the zone plate to block any transmission of the waves. Align the microwave transmitter and receiver to ensure a strong signal is transmitted. Place the zone plate in front of the transmitter.
Turn on the transmitter and reciever if it is not already. Place the fresnel zone plate in front of the transmitter with the cover plate on also. When this happens, the audience should no longer hear the transmitted signal. Removing the cover plate will allow the zone plate to diffract the incoming microwaves. By moving the zone plate along the axis between the transmitter/receiver, you can locate the image position(s).
Traditional lens material works great for refracting visible light, but the refractive index is not the same for the entire electromagnetic spectrum. For example, X-rays are only weakly refracted by glass and other transparent materials. Zone plates get around this issue, since the plate does not need to be transparent to focus the rays. The same zone plate will focus light of many wavelengths to different foci, which allows this to be a type of filter.