It all happened so quickly: you just approached your car after a quick stop at the market, when a tall, slim-built figure pushes you out of the way, breaks your car window, and grabs the iPhone you accidently left on your car seat. You remember the glass, as there was so much of it — tiny bits of luminescent glass cubes spray into the air, which envelope both you and the burglar as he scurries down the crowded sidewalk. To your relief, the car jacker (an amateur) is caught and brought into the station. You didn’t get to see his face which was covered by a ski-mask. Still, investigators have one major clue that can help prove it was him – the tiny glass bits tucked inside his woolen sweater.
Shattered glass can be a powerful forensic tool. The glass particles can shatter onto anything that falls within a four meter radius. Only freshly broken glass found on the alleged burglar matters, and when that happens, it can serve as strong evidence. Of course, how long a morsel of glass stays on a person depends on what they’re wearing. Glass easily traps inside wool, hides inside pockets and buries inside shoe soles. If you’re a budding forensic analyst (and assuming you want to be prepared in the unlikely scenario this ever happens to you), here are the three key steps to follow when looking at glass with a scientific eye. We’ve covered some of the basics before, if you’d like to get reacquainted with the subject.
Step 1: Taking a closer look at each piece.
Glass fragments can be as tiny as a quarter of a millimeter (about the size of a pencil dot). Using fine tweezers, analysis can pick up a single shard of glass and records things like: its colour (it’s not always clear), diameter and thickness. You’ll want to employ the same basic tools when you begin your own investigation — and be vigilant about the size of each shard.
Step 2: A trick of the light — so how does it bend?
When light hits glass, it slightly bends. This bending of light is called refraction. As a ray of light collides with glass, it rapidly slows down in speed. It does this because the density of glass differs from air. Light rays slow down and refract at specific angles (or refraction index) depending on the density of what its’ hitting – glass, water, or vegetable oil, for instance.By comparing the refractive index of the glass at the scene of the crime to the piece found on a suspect’s jacket, you can figure out if they come from the same origin.
Dip the broken glass into a jar full of a liquid (for example water or vegetable oil). If you peer into the jar, notice whether or not you can still see the glass piece. If the sample appears invisible, then the light ray is not bending, meaning the glass has the same refractive index as the liquid. If you can still see the glass inside the liquid, its refraction index differs.
To find the refractive index of the evidence glass, dip a new piece into a series of beakers filled with different types of liquid (water, vegetable oil and clove oil). By peering closely (you may need a magnifying glass) at the boundary where the glass meets the liquid, notice if you see any bright lines. These are called Beck lines and can range in intensity (on a scale from 0 to 5). These Beck Lines are characteristic of the refraction of each glass. By comparing these observations to a standardized list of known refraction index of various glasses, you can find out the exactly what type of glass found on the alleged burglar.
Step 3: The chemical composition.
For this, you need to use a more refined version of a microscope, called a scanning electron microscope. It zooms into the glass piece at such a high resolution that it can figure out its chemical composition. Understanding the chemical composition of the glass can help analysis answer questions like: what manufacturing method was used? What is the shape of the glass – flat or curved? This can help you find the source of the glass, whether a window pane or a drinking glass. A scanning electron microscope can make it easier to figure out the shape of miniscule glass shards. The overall shape of the glass piece also gives information about its source. For instance, toughened glass in some car windows and doors shatter into small cubes. If that’s the 3-dimensional shape in your crime sample, you may have found your car jacker.
Your Turn: Did we forget any other characteristics of glass that should be tested — and do you want to share additional techniques on how to do so? Let us know in the comments. We’d love to hear from you.