Creating Maltese Cross Polarization
Patterns with Birefringent Materials
Jasmine Garani and John Noé
Laser Teaching Center
Department of Physics and Astronomy
Stony Brook University
Introduction
This project was motivated by an interest in polarized light and the way that
polarized light can reveal the internal structure of certain solid materials,
such as crystals and polymer plastics. Light is normally unpolarized, meaning
that the electric field of the light is oscillating in every direction
randomly. Linear polarized light can be created by passing unpolarized light
through a a linear polarizer. The resulting light has an electric field that
only oscillates in one direction. One other type of polarized light is
circularly polarized light. When linearly polarized light is passed through a
quarter wave plate, circularly polarized light is created. The electric field
of this type of light oscillates in a circle. Human eyes cannot percieve
polarized light, but by using a another linear polarizer, the effects of the
light can be seen. If one linear polarizer is positioned perpendicular to
another one (crossed polarizers), extinction (darkness) is created. If instead
the polarizers are put in parallel (parallel polarizers), then nothing with
change and light will still come through.
Our project focused specifically on studying and creating what is called the
Maltese cross pattern, which can be formed with certain birefringent materials.
Birefringence occures
when a material has two indicies of refraction either in the linear directions
(linear birefringence), or radial and azimuthal (radial birefringence)
directions. The Maltese cross pattern can be seen when a material is placed in
between two polarizers and is radially birefringent. We increased our
understanding of the Maltese cross pattern by studying it while using both
linear and circular polarized light, and also creating it using pieces of birefringent
plastic.
History
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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. |
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Examples of the Maltese Cross
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| 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. |
Creating a Radially Birefringent Material
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).
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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
lighter.
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.
