Electromagnetic waves consist of rapidly alternating crossed electric and magnetic fields. The radiation from most astronomical objects is such that the direction of the electric or the magnetic fields in the waves varies in a random manner. We say that the radiation from these objects is unpolarized. Some objects, however, emit waves which have these fields preferentially oriented in one direction. We then say that the radiation is partially polarized.

To determine the direction of polarization in an object we need a way of selecting the radiation polarized in one particular direction. At radio wavelengths, the basic antenna which is used at the focus of a radio telescope to pick up the radio signals is a dipole. It will only pick up radiation with the electric field parallel (polarized) along the length of the dipole. You may have noticed that in some localities the television antennae on houses are horizontal while in others they are vertical. This is because it is essential for the receiving antennae to be polarized in the same way as the transmitting antennae. Astronomical objects may have their radiation polarized in any direction. The astronomer can determine which direction this is by rotating the receiving dipole until the signal output is at maximum. Then the direction of electric polarization is parallel to the length of the dipole and the magnetic field of the radiation is then perpendicular to this direction.

At optical wavelengths there are a number of substances which respond differently to different radiation polarizations. Only one will be described here – Polaroid sheet. This consists of a plastic material in which are embedded a large number of extremely fine needle-like crystals. The polarized needle crystals are arranged so that they all lie parallel within the plastic substance. Radiation which is polarized parallel to the needles can pass through almost without loss. Radiation polarized perpendicular to the needles is blocked. An analogy can be made with throwing sticks at a fence made out of vertical poles. If the stick is thrown so it is horizontal it will not go through the fence whereas if it is vertical it has a good chance of passing straight through. In this way Polaroid sheet is able to sort out the sense of polarization in the optical radiation incident upon it. You can easily check this with a pair of Polaroid-type sunglasses, especially if you look at light reflected from the surface of a pond of water. Only radiation polarized parallel to the surface of the pond is reflected. Polaroid sunglasses block horizon¬tally polarized light when worn normally. If you rotate the sun¬glasses by ninety degrees this light will be transmitted. By rotating the Polaroid sheet and noting where the maxima and minima occur, we are able to determine the polarization properties of light. This is essentially what the astronomer does at the telescope when he wishes to measure the polarization of an astronomical object.

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