![]() The whole ensemble of radiating atoms behaves like one enormous extended dipole source. Visible radiation emitted from a gas laser source is usually plane polarized and coherent because the dipole moments on the radiating atoms in the laser plasma tube are parallel to one another and are locked in phase by the standing optical wave in the laser cavity. The light that gets through the filter consists of those pulses for which the electric vector is mainly oriented along the poorly absorbing axis of the crystals which make up a polaroid filter. the polaroid material exhibits anisotropic absorption. It works because a polaroid filter preferentially absorbs light whose electric vector is parallel with a particular direction, i.e. A polaroid filter can be used to produce linearly polarized light from such an ensemble of randomly polarized pulses. Unpolarized light consists of a collection of many pulses in which the orientation of the electric vector from pulse to pulse is random. The pulses from the various atoms are uncorrelated in phase moreover, the dipole moments on the individual atoms are oriented at random and so the orientation of the electric vector of the emitted light is also oriented at random. Each pulse is emitted from an atomic dipole oscillator that has been set into motion by thermal agitation. The light emitted from such a source consists of a superposition of pulses each of which is quite short on a human time scale, ∼ 10 −8 secs., but quite long compared with the period of the radiation, ∼ 10 −14 to 10 −15 secs. Visible radiation from a hot filament or from a hot plasma is usually unpolarized. ![]() Such radiation is produced only by special sources. The production of elliptically polarized radiation requires the superposition of two plane waves whose frequencies are identical, whose phases are correlated, and whose electric vectors are not co-linear. Further increases in the phase angle produces left hand elliptically polarized radiation. Upon further increase in phase angle, the ellipticity decreases until the radiation becomes linearly polarized again for \(\phi\) = \(\pi\): the plane of polarization is rotated 90 ◦ relative to the plane of polarization illustrated in Figure (9.3.4). As the phase angle between the two electric vectors, \(\phi\), increases from zero, the ellipticity of the radiation increases and the principle axes of the ellipse rotate until they coincide with the co-ordinate axes at \(\phi\) = \(\pi\)/2. However, the principle axes of the ellipse are not parallel with the x,y co-ordinate axes. The sense of the rotation is such that a nut on a right handed screw would advance along the positive z-axis the radiation is right hand elliptically polarized. The tip of the electric vector traces out an elliptical path as time goes on.
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