Polarised Light

Use filters and plastic pieces to turn white light into a rainbow of colours. The plastic pieces will look as if they are made of stained glass. Try looking at your glasses under the light to see colourful stress patterns.

Explanation

The light coming up through the table is polarised light. Polarised light is made of light waves that all vibrate in the same direction.

Some materials cause the polarised light to split into two parts that travel at different speeds. When the split light recombines, it appears coloured. You see different colours depending on the thickness and internal structure of the materials, and the orientation of the polaroid filter. Lots of colours in one area show stress in the material.

Extras for Experts

Unpolarised light is made of up electromagnetic waves. If you looked straight down a beam of light and could see the waves, they would be radiating out from the beam in all directions. We call these waves the ‘components’ of the light beam.

A polarising filter inside the table filters some of the components so that it is becomes plane-polarised. This means that all the components are vibrating the same direction. The polarising filter does this by cutting out one component of light, and also by transforming (or re-directing) the remaining components of light so that they are all vibrating the same way.

Imagine that the filter is oriented so that it cuts out the vertical component of light (the light that is vibrating at a 90° angle to the horizontal). What about all the components that oscillate between 0° (the horizontal) and 89° (almost vertical)? These components are transformed. This means that they are all altered so that they vibrate on the horizontal plane, creating the plane-polarised light seen through the table. The size of their waves are reduced as they are transformed, resulting in the slightly darker view we see through the polarising filter.

When you place a second polarising filter over the table and rotate it, you will see light diminish as the second filter transforms the components of light. When no light comes through, this means that your filter is oriented so that it is absorbing the components of light through the table.

Interestingly, if you rotate a filter in between your blacked-out filter and the table, some light will come back! This is because the middle filter is transforming the polarised light enough (changing its orientation) so that it is no longer oriented in a way that is absorbed by your top filter.

The colours you see come from all the colours contained in white light. Each colour is polarised by the first filter and then split into two perpendicular components as it enters the plastic/glass objects on the table. Each component of light travels at a different speed through the material. When the components exit the material, they recombine, but their wave peaks are out of step with each other (how out of step depends on the thickness and composition of the material).

When the two components recombine, they are polarised in a slightly different direction according to how out of step they were. Each colour is also polarised by a different amount. The second polarising filter blocks certain colours according to their polarisation, meaning that you see different colours as this filter is rotated.

Sometimes you see many different colours in one place on a piece of plastic or glass. This shows that the plastic/glass contains lots of disordered arrays of molecules in this area that cause light to be polarised by different amounts. This shows an area of stress.

Things to Try

If there is a pair of prescription glasses on hand, look at them through the filter.
If you have a pair of polarised sunglasses, rotate them over the light table to see the polarisers at work.
Try crossing two polarising filters at 90 degrees to each other in white light to black out the light. Now insert a third filter between them and rotate it until the light appears again. When the two filters are at 90 degrees to each other, they cancel each other out, as one filter removes the particular components of light that the other would let through. When you have three filters at work, the central filter alters the orientation of light enough so that some is oriented in a way that gets through the third filter. You will notice that the light that gets through is not as strong as the original source of light.
Bring breakable clear plastic forks/spoons to the exhibition and bend them over the table until they break. You can watch the stress form and see that the forks/spoons shatter at those points.

Questions to Ask

Do you know how sunglasses work to cut out glare?

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