Does anybody know a way to create (in laboratory, but with no super-sophisticated equipement) a beam of light with partial linear polarization whose degree of polarization can be adjusted (at least over the range 0-0.5)?
Start with a narrow beam of unpolarized light (e.g. an incandescent bulb shining through a small hole.) Put a sheet of ordinary window glass (or a glass slide or whatever) into the beam, and I'm going to talk about the light transmitted through the glass. If the glass is normal to the light beam, the transmitted light beam will be unpolarized. Then rotate the glass so the light goes through at a shallower and shallower grazing angle. The transmitted light gets more and more linearly polarized. At the shallowest (most grazing) angle possible, 31% of the transmitted intensity is one polarization, and 69% is the other (if I calculated correctly), so the light is 38% linearly polarized. [Don't forget, in your experiments, that the overall intensity of the transmitted light also changes as you rotate the glass.] The nice thing is that the transmitted beam stays in more or less the same place as you rotate the glass, which is convenient for an experiment. The beam only shifts a little bit due to the refraction. If you put two identical pieces of glass in the beam (one after the other) and counter-rotate them, then the beam stays in exactly the same place (the refraction shifts cancel out) and you can get more polarization as well (up to 67% polarized if I calculated correctly).
If you need more polarization than that, of course, you can use the beam of light reflected off the glass, which goes from unpolarized at near-normal incidence, to 100% polarized at the Brewster angle.
Start with unpolarized let. Let it scatter off a surface at a variable angle.
You could also try scattered sunlight (i.e., light from the blue sky). The degree of polarization depends on the angle between the direction of the Sun and the direction the light is coming from. Apparently under the right circumstances you can get up to 75% polarization this way..