Portable Transmission Polariscope

Many physical phenomena exist around us all the time, but remain invisible unless observed with a specialised instrument. Mechanical stress and strain is an example of such phenomena. They play an extremely important and fundamental role in design and engineering. Hence, I wanted to create something that could allow people to better visualize stress and strain. Through making a compact and portable polariscope.

Material:

1. 6mm rod
2. rubber tape
3. glue/epoxy
4. 8mm lead screw with nut
5. 77mm camera polarising filter
6. polarising filter film
7. LED
8. m3 screws
9. m5 screws

Tools:

1. 3d printing
2. tool to cut rods (I used an angle grinder)
3. Allen key for m3 & m5

The transmission polariscope works by passing polarised light through the transparent object and analyzing how the light is altered by internal stress. The light from a source is first passed through a polarising filter, producing a linearly polarised light. This light then travels through the material, and if there are stresses or deformations, it will slightly rotate the light. The light will then pass through the second polariser (the analyser), which is oriented at 90 degrees to the first.

Some other useful things to understand are what birefringence is and what it means. Birefringence is an optical property where a material causes light to split into 2 components that travel at different speeds. Normally, light in a transparent material travels at the same speed in all directions ( called optically isotropic). A birefringent material, such as acrylic, under stress is different as light polarised in one direction travels at one speed and light polarised perpendicular to it travels at a different speed. Due to these 2 different speeds, they emerge out of phase with each other.

Many plastics, like acrylic, are not birefringent when unstressed, but when stress is applied, the material is mechanically stressed, causing the refractive index to become different along different stress directions. Called stress-induced birefringence. The phase difference caused by birefringence makes different wavelengths experience different phase shifts. Some interfere constructively (bright) and some destructively (dark).

The color is determined by the stress optical coefficient, which tells you how strongly a material converts mechanical stress into optical birefringence.

For example, for the same applied stress:

1. A material with a high stress optical coefficient produces a large phase difference - strong colors with high fringe contrast
2. A material with a low stress optical coefficient produces weak birefringence - faint or no colors

Sorry, that was quite long, but you might find it useful later on during observations. If you read all of that, you get a smiley face :)

Source of photos (u might find the info in the sources useful as well) (˶˃ ᵕ ˂˶)

(1)

THE POLARISCOPE. Mu.edu. https://academic.mu.edu/phys/matthysd/L1980129.htm (accessed 2025-12-29).

‌(2)

https://www.researchgate.net/figure/Schematic-of-the-plane-polariscope-used-for-the-pull-out-test_fig1_322129241

I designed everything using Fusion 360. Where the red colored components are meant to be 3d printed

I designed a lot of the tolerance to be quite big, as my 3d printer is not very good with tolerances. But all the parts still turned out quite good.

140mm lead screw

6mm rod:

1. 100mm
2. 90mm (x4)
3. 115mm (x4)
4. 125mm (x2)
5. 50mm (x2)

There are different modules designed to be slid in along the rods. Each module is either essential to the set, like the polarising filter holders, or is used to demonstrate something like compression from screw.

For the LED holder, I just had a mini array of led laying around, so I made a case surrounding it that could fit into the step up.

For the polarising filter, I just slid the cut-out of the polarising filter into it

As for the clamp, I just fitted everything according to the photo

The assembly is basically just sliding the individual modules in the right order

The intensely bright area directly below the screw corresponds to a region of high stress concentration, which is where the screw is entering from and pressing upon the piece of acrylic. This produces a cone-shaped stress field, which is why the light appears to radiate outward rather than forming uniform bands. The gradual fading away from the screw shows how stress diminishes with distance from the load application point

Here, a very thin piece of wrapping plastic is used, either polyethylene or polypropylene wrapping film. The bright yellow region close to the left clamp indicates a large stress concentration where the load is transferred into the film. Thin films cannot distribute stress as evenly as thicker solids, so most of the deformation occurs near the gripping point. producing a sharp gradient in strain, which is why the colour is intense and confined to a narrow region. The narrowing of the strip near the left side shows necking, a classic trait of plastic deformation in polymers.

As the material was stretched beyond its elastic limit, some of the birefringence was frozen in. Even if the load were removed, parts of the film would remain optically anisotropic (behave differently depending on direction) due to permanent molecular orientation.

A reason the thin plastic film appears as a different color compared to the acrylic is due to it being thinner.

This is a piece of plastic I found lying around and decided to observe it under the polariscope. As you can see, even without pulling or pushing it, it already has fringes in it. This is a classic case of residual stress from manufacturing. You can see that the fringes/patterns seem to be propagating from a single point, that point being where the plastic was flowing through during injection molding. During manufacturing, the molten plastic flows outward into the mold. From this point, the molecules become oriented in the flow direction, and then cooling happens immediately from the mold walls. The combination of these steps results in a radial stress pattern.

You can see patterns without interacting with the piece of plastic, as there is locked-in stress due to plastic cooling and solidifying from the outside inwards. Where the outside layers cool first and shrink, then the still hot inner regions cool later and shrink differently.

Here are some other cool observations of things I thought looked interesting.

For the hourglass, you can see small stress around the neck.

For the hot glue, stress appears only inside a certain diameter, then beyond that, there is no locked-in stress. I think this might be due to the manufacturing process, where it's extruded and not moulded.