Dichroism or selective absorption

A dichroic polarizer absorbs the $\vec{E}$ component perpendicular to the transmission axis. Unpolarized light gets converted to polarized light. A wire mesh polarizer is an example. Consider unpolarized light incident on a wire mesh as shown in Figure 16.4.

Figure 16.4: A wire mesh polarizer

The $\vec{E}$ component parallel to the wires in the mesh sets up currents in the wires and this component is not allowed to go through the mesh. The perpendicular component is allowed to go through unaffected. The light that emerges is linearly polarized perpendicular to the wires in the mesh. It is relatively easy to construct a wire mesh with a few ${\rm mm}$ spacing, and this is a very effective polarizer for radiowaves and microwaves. Microscopic wire meshes produced by depositing gold or aluminium act like a polarizer for infrared waves.

Figure 16.5: A dichroic crystal

There are dichroic crystals like tourmaline. These crystals have a preferred direction called the optic axis. For incident light the $\vec{E}$ component perpendicular to the optic axis is strongly absorbed, the component parallel to the optic axis is allowed to pass through as shown in the Fig. 16.5. The light that emerges is linearly polarized parallel to the optic axis. The absorption properties of such crystals have a strong wavelength dependence whereby the crystal appears coloured.

In 1938 E.H. Land invented the polaroid H-sheet, possibly the most commonly used linear polarizer. A sheet of polyvinyl alcohol is heated and stretched in a particular direction causing the long-chained hydrocarbons to get aligned. The sheet is then dipped into ink that is rich in iodine. The sheet absorbs iodine which forms chains along the polymer chains. These iodine chains act like conducting wires and the whole sheet acts like a wire mesh. The $\vec{E}$ component parallel to the chains is absorbed, and the light that passes through is linearly polarized in the direction perpendicular to that in which the sheet was stretched.