A few years ago, I remember watching a CSI: NY episode where thieves were stealing expensive jewelry from different jewelry stores, and a peculiar problem left the investigators stumped for a part of the episode. The problem had to do with the shattered glass from the glass displays. If a person strikes a glass object, like the casing in the stores or a window, the inside of the glass, which can be seen from a side view of a shard, will leave telltale marks indicating the direction from which the criminal struck the glass. Problematically, the glass from the jewelry stores had no such marks, as if no one touched the glass when it shattered. Eventually the investigators discovered that the thieves, a trio of college physics students, used a high pitch sound generator to create sound waves that shattered the glass without anyone putting a hammer or a fist to the glass directly. The investigators did not realize this at first, even though they'd already found the generator, because it produced sound waves above the range of human hearing. They realized that the mysterious devise was a sound generator when one of them noticed that a dog they found at the scene and had collected for evidence would become aggravated whenever the machine was turned on. When aimed at a piece of glass, the machine caused it to shatter with no telltale marks, and the investigators got one step closer to finding the perpetrators.
Sounds a little bizarre, right? Well, it turns out that studying glass patterns can help forensic scientists more than we realized. A recent study found that when high velocity bullets pass through a thin sheets of glass and Plexiglas, the number of cracks created by the bullet is related to the bullet's velocity. So, in theory, forensic scientists can use a windshield's pattern of cracks to get an idea of how fast a bullet was going. Since not all guns fire bullets at the same speed, and since bullets sometimes get damaged when they hit something, making it difficult to analyze the bullet, knowing the bullet's velocity could help investigators to identify a murder weapon when the bullet itself cannot help them.
While the study is very preliminary, and used controlled conditions not quite reflective of the materials we use and see in everyday life, it does open our eyes to the possibility of using glass from a crime scene to help tell the story.
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