If 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 image at left above, the polaroids are crossed, resulting in minimum transmission. At right above, a third sheet of polaroid is inserted between the crossed polarizers. 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.