Crossed PolarizersAn ideal polarizer produces linearly polarized light from unpolarized light. Two ideal polarizers would eliminate all light if their transmission directions are placed at right angles. Polaroid materials accomplish polarization by dichroism. At angles other than 90°, the transmitted intensity is given by the Law of Malus.
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Polarizer PuzzleIf crossed polarizers block all light, why does putting a third polarizer at 45° between them result in some transmission of light? In dichroic materials like polaroid, the component of the field perpendicular to the transmission plane is selectively absorbed. This achieves a rotation of the plane of polarization, but the mechanism is different from that in optically active materials. In the situation shown, the transmitted intensity can be calculated by applying the Law of Malus twice. If the center polarizer is placed at 45° between crossed polarizers, 25% of the light will be transmitted. |
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Law of Malus
The Law of Malus gives the transmitted intensity through two ideal polarizers. Note that it gives zero intensity for crossed polarizers. |
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