Two picket fence sections with slats and rope to make waves.

In the first part of this demo, two vertical 'picket fence' wood grids are mounted on the table. A rope extends through both and is attached at the end to a lab stand. When the rope is wiggled up and down, the transverse traveling wave is unobstructed and travels all the way to the lab stand. In the next part, a horizontal wood grid is substituted for the second grid. When the rope is wiggled up and down, the transverse traveling wave passes through the first vertical grid, but is blocked by the horizontal grid. No traveling wave reaches the lab stand. The vertical wood grid could represent a crystal that absorbs all light vibrating in the horizontal plane so that only vertically polarized light is transmitted. (The horizontal grid absorbs light in the vertical plane, etc.) Crystals exist, like Tourmaline, that selectively absorb the E field along one axis, but do not absorb so much along the perpendicular. A modern equivalent is the Polaroid, which is made of iodosulfate of quinine needles lined up parallel in a nitrocellulose mastic.