Polarisation is a property of light wherein some waves are selectively blocked based on the orientations of their electromagnetic oscillation. While polarisation is most commonly observed with respect to visible light, microwave light can similarly undergo polarisation. This polarisation can be demonstrated under proper experimental conditions, even if the microwave light itself is invisible to the human eye. Note, however, that polarisation of light is dependent on the wavelength of that light, and therefore materials that polarise visible light will not necessarily polarise microwave light, as microwave light is characterised by much longer wavelengths.
Create a microwave polarising filter by arranging roughly a dozen short lengths of thin metal wire parallel to one another. Spacing should be about 1cm. Affix the wire lengths to a frame so you can move them around easily without disrupting the geometry of their arrangement. Note: Wire lengths should be arranged ONLY parallel to one another. They should not overlap. In other words, the arrangement of the final product should resemble conventional notebook paper, not graph paper.
Repeat Step 1 to produce a second microwave polarising filter identical to the first.
Set up your motion detector. Locate the microwave sensor on the body of the unit--it may look like a small darkened dome. Turn on the motion detector and calibrate it with both of the filters held in front of the sensor. Keep the filters oriented such that the wires of both are parallel and overlap as completely as possible.
Slowly rotate one filter while holding the other one steady. If done correctly, this will trigger the motion detector. This occurs because the filters polarise the microwave light emitted by the motion detector in a direction corresponding to the orientation of the wires. Thus, if both filters are oriented equivalently, polarised microwave light can still pass through. When you rotate one filter, however, the microwave light is now being polarised in two opposing orientations, thereby blocking all of it. This abrupt change in the amount of microwave light passing through the filters to reach the motion detector's sensor triggers the motion detector.