Polarizing

Back to Home page back to Colours

These are photographs of pieces of crumpled cellophane, a transparent material made by stretching, which produces a structural anisotropy. The cellophane was between two polarizing filters which had been arranged to give minimum transmission of light with nothing between them. The paradox is that adding an extra object in the path of the light allows more light to pass. The waves of light behave as if they vibrate at right angles to the flow. Polarizers pick out one direction of vibration. Adding a second polarizer at right angles to the first prevents almost all of the light from passing.

Adding certain substances between the polarizers can change the polarization of the light. In this case bright colours have been produced. Even inserting a third polarizer will increase the light unless it is parallel to one of the other two. This is a very strange effect until you know the explanation.

These effects are based on reduction in symmetry. Many substances have complete symmetry, with no preferred direction. Polarizers have a "grain" and behave differently in different directions. Some substances have no symmetry at all, and can rotate the plane of polarization as light passes through them. Quartz and sugars are examples.

Polarization can be produced by the scattering light from a surface which is not an electrical conductor. Glass and water are good examples. Polarizing sun-glasses are very useful from reducing glare from wet roads when driving towards the sun, and for seeing into the water when fishing. The first picture below shows a trout in the river Coln at Bibury, Gloucesterhire. The photograph was taken without a filter. For the second photograph a polaroid filter was used.

Colours may be enhanced by the use of a polarizing filter on a camera. Skylight is partially polarized. The larger the scattering angle the greater the polarization. This effect can be used to darken skies in photographs, though care is needed when using wide-angle lenses because of non-uniformity. The picture of the Weisshorn in Switzerland shows exaggerated contrast through the use of a polarizer. The picture of the Aiguille de Chardonnet is perhaps not quite as bad. The picture of one tower of the Severn suspension bridge was made using a 28 mm lens with a polaroid filter and 35 mm film. The brightness of the sky increases noticeably from left to right. The reflection from the tower shows the position of the sun.

Here are two pictures, one taken without a polaroid filter, and one with a filter – click on Brewster.

The colours of many subjects can be clarified by the use of a polarizer. This is possible because the reflected light from the sky or the sun has a specular component which is partially polarised. Leaves, for example, often have a shiny surface which reflects well.

By placing a piece of transparent material between crossed polarizers we can sometimes detect variations in thickness, variations in strain, variations in composition, or variations in orientation of crystal axes. This is a perfectly good set-square, the function of which is not at all affected by the effects revealed by the polarised light. We should note that the colours may be influenced by the thickness of the material as well as by the strains, so the interpretation of the pictures is not necessarily obvious.

Here is another set-square. It does not produce colours, but there are clear effects around the large hole and the two rivet holes. The remainder of the instrument showed very weak effects, compared with the set-square shown above.

The next picture shows a CD box. Three other boxes produced very similar pictures. The reproduction process does not do justice to the colours, which have been subjected to tri-colour photography, scanning, JPEG compression, and display on a computer screen.

Here is a box and a bread-bin. Almost any transparent plastic box will give results. In the left hand picture, the point in the middle is the point of injection into the mould. Here are more examples.

In the last three pictures above we see a protractor. The pattern differs completely from all the previous ones. The protractor was probably made from rolled sheet and not by injection moulding. The third picture is a fraud – it was made by reversing the colours of the middle picture.

The next pictures were made using thin plastic films of the types used for wrapping goods.

And here are examples made using food-wrap –

In the example below, a straight thin strip of perspex has been curved by external forces. The colours show the presence of the strains. A calculated picture is shown for comparison. The photograph is dark along the centre line, or neutral axis, showing that the strain there is very low.

Liquid Crystal Displays

In an LCD display, the electric field between the electrodes rotates the plane of polarization of light passing through a liquid crystal. These pictures were made using a linear polaroid filter in two different orientations.

Aurora Borealis?

Not the aurora, just the windscreen of a Peugeot 106, viewed between crossed polaroids.

Back to Home page back to Colours