Polarizers Polaroid

FIGURE 5.7. Light is plane polarized on reflection and is transmitted through a detector of polarization (polaroid shown here) only if the orientation of the detector is correct for the passage of plane-polarized light. This orientation is correct as in (a), but not in (b). The experiment of Malus with candlelight on a bowl of water made use of the same principles, with a calcite crystal replacing the polaroid. 

The electrolyte was a solution of ammonium chloride that bathed the electrodes. Like Plante s electrochemistry of the lead-acid battery, Leclanche s electrochemistry survives until now in the form of zinc-carbon dry cells and the use of gelled electrolyte.12 In their original wet form, the Leclanche electrochemistry was neither portable nor practicable to the extent that several modifications were needed to make it practicable. This was achieved by an innovation made by J. A. Thiebaut in 1881, who through encapsulating both zinc cathode and electrolyte in a sealed cup avoided the leakage of the liquid electrolyte. Modern plastics, however, have made Leclanche s chemistry not only usable but also invaluable in some applications. For example, Polaroid s Polar Pulse disposable batteries used in instant film packs use Leclanche chemistry, albeit in a plastic sandwich instead of soup bowls.1... 

By performing sequential SG experiments it can be shown that electron spin has much in common with photon polarization. In a typical arrangement the beam from the oven is first passed through an SGZ apparatus with the inhomogeneous magnetic field in the z-direction. Suppose the one (S ) component from this experiment is blocked and the second (5+), is allowed to pass into another SGZ apparatus. The beam emerging from the second device will be found to have no S component, exactly as for two parallel polaroids. 

Although laboratory polarimeters generaUy use Nicol prisms as polarizers and analyzers, dichroic crystals (such as tourmaline) or dichroic sheet polarizers (such as Polaroid) may be used in the construction of special apparatus.f... 

These devices are based on the anisotropic absorption of light. Usually molecular crystals exhibit this property and tourmaline is the classical example for this. For practical purposes, however, micro crystals are oriented in polymer sheets. Polymers containing chromophors become after stretching dichroic polarizers. The devices produced in this manner are called polawids. They have found a broad application in many technologies. Their application in spectroscopy is limited to the near ultraviolet and to the visible and near infrared range of the spectrum. In vibrational spectroscopy polaroids are employed as analyzers only for Raman spectroscopy. 

Figure 8 Schematic layout of a typical 90° Raman depolarization experiment showing the positions of the polarization analyzer and the scrambler. The analyzer may simply be a polaroid sheet, which can be rotated by 90° to allow the parallel ( ) and perpendicular ( ) components of the scattered light to pass through to the detector. The function of a scrambler is to change linear into circular polarization of the light entering the Raman spectrometer slit in order to avoid measurement errors due to the variable spectrometer transmittance of the light polarized in different directions...

Polaroid , the material of which most polarizers are currently made, is composed of a clear sheet of polyvinyl alcohol. It is stretched in one direction to align the molecules and dyed with iodine molecules that align themselves along the polymer molecules. This polaroid film will transmit light only in a direction perpendicular to that in which the film was stretched. Polaroid was first made in 1928 by Edwin Herbert Land from quinine sulfate periodide. " The initial idea for this came from the observation of William Bird Herapath that when iodine is dropped in the urine of a dog that has been fed on quinine, shiny green crystals (now referred to as herapathite) with strongly polarizing properties are formed. 

Crossed Nicol prisms An arrangement of two materials (calcite polarizing prisms or Polaroid discs) such that the first, the polarizer, transmits plane-polarized light that passes through the second, the analyzer, only if oriented in specific ways. 

How can this rotation of the plane of polarized light—this optical activity—be detected It is both detected and measured by an instrument called the polarimeter, which is represented schematically in Fig. 4.2. It consists of a light source, two lenses (Polaroid or Nicol), and between the lenses a lube to hold the substanefe that is being examined for optical activity. These are arranged so that the light... 

Polarization-based contrast (211 in which the specimen is illuminated normally with linearly polarized light and the analyzer is replaced by a small disc of Polaroid in the center of the objective back focal plane. This allows any intermediate contrast condition between bright field and dark field to be selected, and appears to be untried in the context of liquid crystals. 

The convenhonal spectrometer consisted of a Coderg double monochromator equipped with a cooled PMT and was described in detail by Brooker et al. (1994). A 1 W laser was required to obtain spectra with adequate signal to noise ratio. A half-wave plate controlled the polarization of the incident beam. The 90° scattered light was analyzed with Polaroid films with accepted parallel or perpendicular polarized light. A quarter wave-plate in front of the entrance slit served to compensate for grating polarization preference. 

A Polaroid filter allows light through only if the light is polarized at the same angle as the filter. 

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