Recently, my extremely talented artist sister Jackie and I began a collaboration to merge our professional passions: Science Pants! Because we’re sassy, we make more than just leggings (tank tops, t shirts, bags, and more!), and we don’t sell anything that we don’t love.
While Jackie is in charge of design, I am responsible for gathering beautiful scientific images and explaining them; our first products show EXPLOSIVES, which look like boring white powders to the naked eye, but become gorgeous when visualized with Polarized Light Microscopy (PLM). In this post, I’ll (hopefully) shed some light on PLM (sorry – couldn’t resist the pun).
PLM is a way to observe both the optical and physical properties of crystals. This technique, also called polarizing microscopy, is used in many fields, including identifying suspicious white powders in forensic investigations, characterizing minerals in geology, and evaluating cartilage structure in medical applications.
Happily, PLM also creates beautiful images of substances that are uninspiring to the naked eye! We used PLM images of various components of organic explosives to create our flagship line of Science Pants. Science Pants let you flaunt explosives like ammonium nitrate (NH4NO3), cholesterol acetate (CA), TNF, dintironapthalene (DNN), mononitronapthalene (MNN), paranitrophenol (NP), picric acid (PA), trinitrobenzene (TNB), and trinitrotoluene (TNT).
The images used in Science Pants were acquired by Jessica Ye, a current MD/PhD candidate at Yale, during her undergraduate chemistry lab at Cornell. She used a Leica DM EP microscope to identify unknown crystals by characterizing both physical and optical characteristics. In the lab, she spread crystal samples on slides, covered them with glass coverslips, and then melted them on a hot plate. She put the samples under the polarizing microscope during and after recrystallization and observed them under 4x, 10x, and 40x magnification. She then integrated various observations and measurements to calculate the refractive indices of ‘mystery crystals’, compared them with a panel of known organic explosives, and correctly identified the mystery samples, earning a top grade on her lab report and a chance to put crystals on Science Pants enthusiasts across the world.
PLM is a powerful technique in that it can identify the vibrational directions in different planes of the crystal. To characterize the directionality of electrons, polarizing microscopes contain polarizers, one above the sample and one below. The first polarizer, beneath the specimen stage, has its vibration azimuth fixed in the East-West direction. The other polarizer (the analyzer), usually aligned with a vibration direction oriented North-South, is placed above the objective and can be moved into and out of the light path as needed. When both the polarizer and analyzer are inserted into the optical path, their vibration azimuths are positioned at right angles to each other; in contrast, most conventional light microscopes shine randomly-oriented light through samples. In this configuration, the polarizer and analyzer are crossed, with no light passing through the optical system and a dark background present in the eyepieces.
Polarizers generate linearly polarized light waves and filter out a privileged plane from the statistical confusion the random vibrational directions of natural light. To ensure that the 2 wavefronts have the same amplitude for maximal contrast, the analyzer recombines components of the 2 parallel wavefronts that are vibrating in the same plane. Refractive index values are generated by the 2 orthogonal components of light (ordinary and extraordinary waves) traveling at different speeds through the specimen. The faster wavefront emerges from the specimen first, creating an optical path difference with the slower wavefront, a phenomenon called birefringence. The birefringence value is the numerical difference between the wavefront refractive indices, and this value is one useful metric to identify unknown substances 🙂