https://www.sciencealert.com/this-high-speed-footage-of-bullets-hitting-unbreakable-prince-ruperts-drops-will-blow-your-mind
(Smarter Every Day/YouTube)
For those of you who aren't familiar with the Prince Rupert's drop, this weird, scientific enigma is a seemingly simple glass object created by dripping molten glass into very cold water.
That process creates all kinds of crazy physical properties, which we'll go into later, but the end result is a teardrop-shaped piece of glass that's practically unbreakable at its bulbous 'drop' end, but will shatter from the slightest pressure at the elongated tail end.
Physicists have been obsessed with these 'unbreakable' glass drops since the 1600s. But what happens if you shoot one with a bullet?
In a 2016 video, Destin from Smarter Every Day used the newest technology to find out, by documenting the entire thing at a glorious 150,000 frames per second.
https://youtu.be/24q80ReMyq0
Spoiler: Prince Rupert's drops are so strong, they actually cause the bullet to shatter.
So, what's going on here? As you can see in the video above, shooting the Prince Rupert's drop will typically make the glass object shatter too, but that's not because of breaking the glass at the thick end.
In glorious slow motion, you can watch as the bullet crumbles against the wide end of the drop, sending out shock waves that then rattle the rest of the structure and cause the thin end to break, resulting in the entire thing exploding.
To understand how this works, you first need to understand why a Prince Rupert's drop is so weird in the first place.
When the Prince Rupert's drop is made, molten glass is poured into extremely cold water, causing the outside of the drop to cool and solidify almost instantly, while the inside remains molten and cools more slowly.
Because of thermal expansion, glass wants to expand while it's hot, and contract while it's cool.
That means that as the molten inside of the glass gradually cools down, it wants to contract and pull the solid outer layer inwards. But because the outer layer is already solidified, this just makes the whole thing tighter, making that bulbous end of the Prince Rupert's drop pretty much indestructible, and, as it turns out, bullet-proof.
But because the outside of the glass is in extremely high compressive stress, and the inside is in extremely high tensile stress, if one link is ever broken, the whole thing explodes, feeding off its stored internal energy.
This is what happens when the fragile thin end at the back of the drop gets broken - it releases all that pent-up energy, and that's why the entire thing shatters.
To understand that properly, check out this incredible slow-mo video that Destin originally did on the Prince Rupert's drop a few years ago:
https://youtu.be/xe-f4gokRBs

Now we know that these incredible pieces of glass are even more fascinating, because they can also withstand a bullet from a .22 rifle when shot at the right angle.In fact, if you watch to the end, you'll see the mark left on one of the Prince Rupert's drops which was only grazed by the bullet, and didn't even break. So epic.
A version of this article was originally published in 2016.
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