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Camera
Fblarizing analyzer
Po
Source First Polarizer
Glass slug Pulsing coil
L_LJ
E(impressed voltage) Fig. 4. Diagrammatic presentation of the principle of the Faraday shutter.
reactions, and the experimental 1-jusec Faraday shutter was developed and loaned to our laboratory to be used in the study of explosives. An analysis of the applicability of that shutter to explosive studies has been made.56
In 1845 Faraday noted that a block of glass in a strong magnetic field displayed optical activity. Further, he observed that when a beam of planepolarized light passed through the glass parallel to the lines of force in the magnetic field the plane of polarization was rotated in accordance with:
VHl
where :
4 is the angle of rotation in minutes of
arc; V is the Verdet constant in minutes of
arc/Gauss/cm (0.064 for the sodium
5893 A line);
H is the field strength in Gauss; and / is the length of the path through the
field in cm.
The application of this effect to the magnetooptic shutter is shown in Fig. 4. The rays from the light source, L, are propagating in the direction D with
random vibration. The polarizing screen P0 passes only the components of vibration shown. After the beam of plane-polarized light passes through the glass slug, which is normally optically isotropic, it is stopped by the polarizing analyzer PA whose axis is perpendicular to the original polarizer PQ. However, when a high-voltage pulse E is sent through the coil of wire which is wrapped around the glass slug, a rotation of the plane of polarization of the beam from PO results (Faraday effect) so that the analyzer PA is no longer perpendicular to the plane of vibration incident upon it. The components of the rotated beam P0<£ in PA pass through to the camera. The duration of the pulse E therefore controls the duration of the passage of light through the analyzer PA to the camera.
The main units of the Faraday shutter can be seen in Fig. 5. It is essential that the opening of the shutter be synchronized with the event so that a study may be made of an explosive reaction at any desired instant. To accomplish this synchronization to within 1 /xsec the Rapatronic shutter is supplied with a
152
February 1953 Journal of the SMPTE Vol. 60