Creating Maltese Cross Polarization
The Maltese cross pattern is now known as the universal symbol of fire rescue. This idea comes from the "Knights of Malta", who wore a cross on their skirts when they fought. Often they had to save their comrades from buring buildings when their foughts against the Saracens, who often used fire as their principle weapon. The "Knights of Malta" became the first recorded fire fighters, so fire services use the Maltese cross cross as the symbol of their profession.
| Dark Maltese cross:|
The two linear polarizers are creating extinction, which is the dark area around the Pringles can cover. The dark cross in the Pringles can cover occurs because at those points the plastic is not affecting the light that is coming through the first polarizer. Where there is color is where the birefringence of the Pringles can is creating a phase shift in the polarized light coming throught the first polarizers, making it so the polarization is now in a differenct direction, so exctinction will not occur when the light passes through the the second polarizer.
Light Maltese cross:|
When the second linear polarizer is turned 90°, the two polarizers are parallel and there is no extinction. Once again, there are certain areas where the plastic does not affect the light that is coming through. This creates a light Maltese cross pattern.
Circularly polarized incident light:|
Unpolarized light passes through a circular polarizer consisting of a linear polarizer and a quater-wave plate, then through the pringles can cover, and through a linear polarizer. Because the electromagnetic field in circularly polarized light is oscillating up and down and rotating, the same Maltese cross pattern does not appear. However, when either the linear polarizer or the can cover is turned, the pattern does not change.
To create a "synthetic" radially birefringent material I took a piece of plastic used for overhead projectors and cut it into triangles (below left). I arranged the triangles into a circular pattern (below right) and used small pices of Scotch tape to hold the pattern together (pictures).
When the plastic circle is put in between crossed polarizers, four of the triangles show the extinction. The
other four interfer with the light coming through the first linear polarizer, so the direction of the
polarization is changed. These triagles do not show extinction, and are therefore
When the top polarizer is turned 90° the polarizers are parallel and no extintion occurs. These two pictures show that the plastic films made for overhead projectors are birefringent and can be arranged to create a Maltese cross pattern.