The photo above is soapy water sprayed as a thin film on plexiglass, frozen, and shot using cross-polarized light. Without crossed polars, this looks just black and grey.
While looking at some beautiful photomicography (pretty much the same as what I do, only using a microscope instead of foolishly hand-holding the camera like me), I stumbled across a way of using two polarized films to highlight details in a subject typically invisible to our eyes.
Your sunglasses may be polarized, and you would know because when you look at a shiny car in the sun, the reflections change noticeably. This is because the polarized film of your sunglass lens is only allowing light at one angle to pass through. To try out this concept, I simply used the two lenses of an old pair of shades.
If you take these two lenses out of the frame and hold them up against eachother, stacked, you will notice that things look just a bit darker through them. But, rotate one of them 90° from the other, and you will notice you see nothing through them. Like magic, they turn black against one another.
In photomicography using crossed polars, one places their light source directly beneath the subject, with a polarized film in between them. In this case, one of my sunglass lenses.
The other sunglass lens is situated between the subject and my camera lens (called the "objective" in fancy microscope-jockey terms), and I simply used tape to hold it in place over the front of my lens. It is important when shooting this way, to hold the camera and position the lens so that the two polarized films (sunglass lenses) are at 90° angles to eachother. They should "black out" eachother, but you can still see the subject between them.
If your subject is plastic, or a crystal structure, you should see rainbow patterns inside them as you dial in the correct angle of the lens/polarizer relative to the light/polarizer. And this is how it's done. I want to reiterate that all these photos were made using white light, no colored lights or fancy computer tricks involved.
Scientists shoot minerals and other substances using this technique (though far more complex than I have explained here) and are able to identify materials by the way they scatter light between the two polarizing films. I am just using it to make pretty pictures.
Above is another view of the frozen soapy water. I think it was starting to melt at this point. The three last images below are different views of bubble wrap using cross polarized light.
