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Q & A: Strain viewer colors

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Most recent answer: 10/22/2007
Q:
in a strain viewer apparatus how do we get a magenta background?
- kamal
iit delhi, india
A:
It sounds as if the kind of strain viewer you are asking about is the optical kind. These consist of two polarizing filters, one of which can be rotated relative to the other. In between the two filters is where you put the object whose strain is to be measured. Only certain kinds of objects will do -- they have to be clear enough to let the light through, and the strain in the object has to affect the polarization in some way.

When the two polarizing filters are arranged so that their directions of polarization are perpendicular to each other, very little (ideally, zero) light passes through both filters. One filter will let through light waves which have their electric fields pointing, say, vertically, while the other polarizing filter only lets through the part of the waves where the electric fields point horizontally -- no light wave can satisfy both. If the first filter is rotated a little bit, then some light will get through the second filter, as a small component of the electric fields will point in the horizontal direction. Quantum mechanically, we say that a small fraction of the photons pass through.

Some materials can rotate the plane of polarization of light that passes through. Common plastics, such as cellophane, do this particularly well. Liquid crystals also rotate the plane of polarization, and change their behavior when an electric field is applied. Liquid crystals, along with polarizing filters, are used in LCD displays. Have a look someday at a liquid crystal display with polarizing sunglasses -- it's fun!

The material whose strain is to be investigated is usually made out of a clear plastic that rotates the plane of polarization of polarized light going through it. The amount of this rotation depends both on the strain in the plastic, and on the color of the light. By adjusting the polarizing filters and the amount and strain of the plastic, you can get a desired color to appear. You even see this effect on sunny days when looking at the glass windows of cars with polarizing sunglasses. Most car windows has a sheet of plastic inside to keep the glass shards from going everywhere when the window breaks. It is pressed there under high stress and has an interesting deformation pattern (usually a grid of spots). You can often see these spots in different colors with polarized glasses on a sunny day.

To get a colored background other than the gray that's usual for these things, I'd suggest putting a thin layer of cellophane between your polarizing filters. It may not be the best, since most cellophane's thickness isn't controlled all that well, and there may be stretching. Try different kinds -- food storage wrap, freezer bags, and the clear plastic wrap found around many boxed items. Try rotating the polarizing filters until you get the color you like. More than one thickness of cellophane may be needed -- experiment around!

You can also "cheat" and get a magenta filter for your light source -- you'll be sure to have magenta light if you keep the filter (especially if it is made out of plastic) outside of the space between the polarizing filters. If the filter is between the two polarizing filters, then any strains in your colored filter may appear as colored stripes in colors you may not be that interested in.

Good luck!

Tom

(published on 10/22/2007)

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