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.
The 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 
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.
There are dichroic crystals like tourmaline. These crystals have a
preferred direction called the optic axis. For incident light the
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 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.