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Polaroid Filter and the Transmission Axis:
A Polaroid filter is said to have a transmission axis. The filter can be thought of as having long-chain molecules that are aligned within the filter in the same direction. Unpolarized light can be thought of as consisting of waves with electric and magnetic components of vibration that are directed in a multitude of planes. The electric and magnetic components that are aligned in the direction of the molecules become absorbed by the molecules. Those components that are perpendicular to the molecules emerge from the filter. Thus, the transmission axis is aligned perpendicular to the molecules of the filter. One-half of the original vibrations emerge from the filter. The emerging vibrations are aligned in a direction perpendicular to the molecules and parallel to the transmission axis.
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The diagram shows unpolarized light entering two consecutive filters. The transmission axes of both filters are aligned parallel to each other - either both vertical or both horizontal. Unpolarized light will enter the first filter and exit as polarized light. The plane of vibration for the light exiting the first filter will be in the same direction as the transmission axis of the second filter. Thus, the second filter will have no effect upon the light; after all, the light that enters the second filter is already polarized in the direction of the transmission axis. The combined effect of both filters is the same as the effect of the first filter - to filter out one-half of the vibrations and align the remaining vibrations parallel to the transmission axes.
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Polarization by Transmission Through a Filter:
A Polaroid filter is a device that polarizes light. The filter serves to filter out one-half of the vibrations of unpolarized light as the light passes through the filter. Unpolarized light enters the Polaroid filter and polarized light exits the filter with one-half the original intensity and with vibrations in a single plane. The unpolarized light becomes polarized.
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