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Posts Tagged ‘clear plastic’

Photoelasticity and you

Stress

Stress

You probably already know that polarized lenses only allow light to pass through in a particular orientation1 2 You may already know that if you take two polarized lenses and line them up so they are both horizontal you can see through them fine, but as you rotate it so one of them is 90 degrees from the other3 suddenly the second lens goes dark.

What you may not know about is photoelasticity.  If you take a clear piece of plastic and put it between two polarized lenses with a 90 degree orientation, you’ll get to see something very cool.  You will actually be able to see the stress marks within the clear plastic as light, dark, and colored bands.  What’s happening is the slight density variations in the clear plastic are very slightly altering the orientation of the light passing through – making some parts darker and other parts lighter as they are oriented towards or away from the secondary lens.

If you try the same trick, placing a clear piece of plastic between two polarized lenses, and flex the clear piece of plastic you’ll actually be able to see how the plastic responds to the application of force – as you watch stress patterns develop across the plastic.  This process is sometimes used by architects on clear plastic models to better understand how load stress is distributed throughout a building.

I recently obtained several polarized lenses and am very curious to see how my MK4 and MK5 plastruders look through them.

  1. Say, either horizontal or vertical. []
  2. Photo courtesy of drammatico []
  3. Essentially so that one only lets through vertically oriented light and the other only lets through horizontally oriented light. []
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