User:Dave3457/Sandbox/Polarized Filter

Creating Circularly Polarized light
One way to create circularly polarized light is to place a quarter wave plate after a linear polarizer. The transmission axis of the linear polarization needs to be half way (45°) between the fast and slow axes of the quarter wave plate. Technically speaking, one can think of the single component of linearly polarized light as divided into two separate components at right angles to each other, of equal amplitude and of the same phase. The orientation described above results in one component being parallel to the fast axis of the quarter wave plate and the other parallel to the slow axis. After passing through the wave plate the components are still equal in magnitude but their relative phases have changed by a quarter wavelength. The result is that their vector sum corresponds to circularly polarized light.

To appreciate the process in a more concrete at the 45 degree angle. The single electric field of that light can be imagined as being composed of two separate components, one vertical and one horizontal. They will be of equal magnitude and in phase. While in the wave plate the vertical component of the light, which is along the slow axis of the wave plate, will travel at a slightly slower speed than the component that is directed along the horizontal fast axis. Initially the two components will be in phase, but as the two components travel through the wave plate the vertical component of the light will drift farther behind of the horizontal component. By adjusting the thickness of the wave plate one can control how much vertical component is retarded relative to horizontal component before the light leaves the wave plate and they again begin to travel at the same speed. When the light leaves the quarter wave plate the vertical component is exactly ¼ of a wavelength behind of the horizontal component making the light circularly polarized. To appreciate how the “re-integration” of the two components of light implies that that new electric field traces out a screw type pattern in space one needs to appreciate that the two new components are not in fact separate entities but in fact, at every point in space, add together to produce a single electric field. To appreciate how they vectorially add to produce this screw type pattern, imagine a dot traveling counter-clockwise in a circle with each position of the dot relative to the center of the circle corresponding to a different point long an axis in the direction of the wave. Just as the vertical and horizontal displacements of the dot, relative to the center of the circle, vary sinusoidally and are out of phase by one quarter of a cycle, the magnitude of the vertical and horizontal components of the electric field of circularly polarized light vary sinusoidally and are out of phase by one quarter of a wavelength. In the accompanying diagram, the vertical component is red and the horizontal component is blue. In the instance just sighted, the light will be clockwise circularly polarized, as seen from the receiver, which is to say a receiver, watching the light approach, him will observe the electric and magnetic field vectors rotate it the clockwise direction. This light is also considered left hand polarized because the electric field forms a left handed helix in space. The above conventions are the ones used in physics, astronomy and optics. To create counter-clockwise circularly polarized light one can simply rotate the axis of the quarter wave plate 90 degrees relative to the linear filter. This reverses the fast and slow axes of the plate relative to the linear filter. Absorbing and passing circularly polarized light

Circular polarization filters can be use to selectively absorb or pass clockwise and counter-clockwise polarized light. It is this feature which is utilized by the glasses of modern 3D cinemas such as RealD. A filter which creates a given polarization of light will, when the direction that the light passes through the filter is reversed, transmit that same polarization of light and block the opposite polarization. To understand why, again imagine the concrete example cited above. While the linear filter transmits light if its electric field is at the positive 45 degree angle relative to the right horizontal, it absorbs light if its electric field is perpendicular to that angle. Imagine clockwise polarized light, approaching the quarter wave plate from the other side. Again the screw like pattern of the electric field can be thought of as divided up into vertical and horizontal linear components. The vertical component will be one quarter of a wavelength ahead of the horizontal component. After passing through the quarter wave plate, which has its slow axis in the vertical direction, the two components will be in phase, the result will be light linearly polarized at a positive 45 degree angle relative to the right horizontal. As a result, it will pass through the filter. When counter-clockwise light passes through the quarter wave plate, the horizontal component will again be retarded, but the angle of the resulting linearly polarized light will be on the absorbing axis of linear filter. To create a filter that instead passes counter-clockwise polarized light and absorbs clockwise, one rotates the quarter wave plate 90 degrees relative to the linear filter which reverses the fast and slow axes of the wave plate.

Polarizing filters used in Photography
A polarizing filter, used both in color and black and white photography, filters out light polarized perpendicularly to the axis of the filter. This has two applications in photography: it reduces reflections from some surfaces, and it can darken the sky.