Microwave Optics Physics 470
Fall 2011
Purpose
Microwaves display a variety of optical phenomena similar to those of visible light. This lab extends the study of optics by using microwaves to demonstrate some optical phenomena which may be difficult to see with the shorter wavelengths of visible light.
Method
The method of this lab will be to set up a demonstration of an optical effect using microwave transmitters and receivers, and various objects which are analogous to optical elements used in the visible region. In some cases, it will be instructive to collect enough data to see the similarity between the microwave results and the optical results.
Procedure
The procedure will be to set up each of these one by one and collect appropriate data. Explain the results in terms of similar optical phenomena. See any upper-level undergraduate Optics book for review (authors include Hecht, Pedrotti, and/or Moller).
Polarization by wire-polarizer
Polaroid polarizers are sometimes said to be like wire polarizers, although the microscopic mechanism is quantum mechanical to some extent. In the microwave range, we can make true wire polarizers which can be explained using purely classical EM theory. The results for the intensity of transmitted microwaves should look much like Malus’ Law, which states that I = Io cos2, where Io is the incident intensity of plane-polarized light, I is the transmitted intensity after a linear polarizer, is the angle between the plane of polarization of plane-polarized incident light and the transmission axis of a polarizer. Unlike the case for your optical experience, the transmitter and receiver in the microwave lab are horn antennas, which polarize the beam (so the transmitted beam is already polarized). The receiver is also sensitive to polarization, so it behaves like a polarizing element followed by a detector.
Set up a transmitter and receiver and investigate the angular dependence of the intensity as you rotate the receiver. Try putting the beam through one of the grill-shaped reflectors or wire polarizers and see if you can reproduce the behavior described by Malus’ Law. Plot your results vs. the relative angles of polarizer and horn.
Diffraction from single and double slits
Investigate the intensity pattern at a distance of 0.5 to 1.0 meters from the single slit and the double slits.
Fabry-Perot interferometry
Use two of the half-reflectors in a parallel arrangement and investigate the transmitted intensity as the two are moved apart. Explain why the intensity might vary as the half-reflectors are moved apart with a periodicity equal to half the wavelength. Determine this wavelength, then calculate the corresponding frequency of the microwaves and compare with the known (or design) value (see the side of the transmitter).
Michelson interferometer
See the description in the printed instructions for the microwave set (in a folder kept with the apparatus).
Mach-Zender interferometer
See the Optics book by Hecht or discuss this with the instructor. A pair of wooden wedges is available to adjust the optical path length in one arm of the M-Z.
Total internal reflection
Use a big block of salt in the form of a prism to demonstrate total internal reflection.
There are also some big plastic lenses, shaped like a half disk. Do these focus the microwaves?
Frustrated total internal reflection
Now place another salt prism near the first to see some transmission into the second. Investigate the transmitted intensity as a function of the separation of the two prisms.
This has been done carefully by a student in the past, and the results are not a simple exponential decay! (Think resonant cavity.)
Some advanced topics:
Poisson spot
See the paper by K. C. Chu (and other references such as an Optics book). Use the round disks to obtain diffraction and look for a peak directly behind the disk.
Crystal diffraction
Use the artificial crystal to diffract beams in various directions. It may be difficult to obtain a Bragg condition. See the write-up from Preston’s book.
Any book on Solid State Physics has background material for this topic.
Goos-Hanchen effect
See the write-up by K.C. Chu to investigate this effect. (This turned out to be difficult.)
Fresnel plate
There are some ring-shaped foil cutouts if we want to try this.
Quarter-wave plate
A block of wood (2x4),which has been cut into two wedges to allow you to vary the thickness, will act as a quarter wave plate for microwaves. It is also possible to set up an interferometer and observe the difference in index of refraction of the wood, which is an anisotropic material due to the grain structure. We have not tried this and I expect that it will be difficult to see anything.
References
Meiners, Physics Lecture Demonstrations
K.C. Chu and John Noble, retired WIU professors, Poisson spot (presented to ISAAPT)
Preston, The Art of Experimental Physics
Equipment
Microwave Optics set