Colorful stress patterns

Tension made visible!

20 minutes


  • Carefully review the general safety advice on the back of the box cover before starting the experiment.
  • Disassemble the setup after the experiment.

Step-by-step instructions

Let’s assemble a simple holder for your polarizing filter.


White paper creates a light background—all the better for observation.


The glasses in your set are made with a similar polarizing filter. That’s why the film in the frame should appear black when you look at it through your glasses.


Stretching the bag doesn’t just change its form—it also changes its optical properties.


Despite the fact that solid plastic objects already have their final shape, we can assume that they were stressed during production. This also changes their optical properties.



Dispose of solid waste together with household garbage.

Scientific description

When two polarization filters  block light completely, we say that they are “crossed.” If we can see a transparent object placed between crossed filters, that means that the light’s polarization changed while passing through it. If you rotate a plastic bag between two crossed polarizers, at some point, light will begin to pass through the polarizers again: the material of the bag is  changing the polarization of the light . These changes depend on the bag’s inner structure.

You can change this structure by stretching the bag, as the molecules straighten in the direction they’re stretched. This also affects the polarization of the passing light: now you should see an array of colors on the bag! The colors appear because differently-colored photons change their polarization differently as they pass through the plastic bag. The brightest hues  are those with a final polarization closer to the polarization favored by the second filter . Plus, different areas of the bag have different tensions, and this creates different colors.

Just like the stretched bag, hard plastic objects become visible and colored between two crossed polarization filters. The thing is that plastic objects have certain areas where molecules are aligned in a certain direction. This sort of alignment is caused by how things are produced from hard plastic. This effect helps us study some internal qualities of plastic objects and determine their possible weak spots.

That’s interesting!

Polarizers are used in sunglasses, in the production of 3D movies, and to eliminate glare in photography! The effect observed in this experiment is used to search for defects in transparent plastic objects.