We’ve all been there at one stage in our lives, one minute we’re standing next to a perfectly fine piece of transparent silica (commonly referred to as glass) and the next we’re faced with a crack which seemingly came out of no where. Whilst we’re often able to see the events leading up to the eventuality the actual mechanism of a crack appearing appears to be instantaneous. Every driver who’s been on a gravel road and had the unfortunate happen can attest to this and whilst it might be of little comfort the process of that line appearing is actually quite intriguing. However you’re not going to be able to see it without anything but highly specialized equipment, something that can recording millions of frames per second.
That video is shot at a staggering 10 million fps which is why the small ball appears to be hanging motionless in mid air for an eternity before the cracks in the glass appear. By comparison the cracks themselves appear to snake out from the point of impact with blazing speed, reaching the outside of the pane in an incredibly short span of time. This is why cracks in glass appear to come out of no where as the speed at which they move is so fast that it’s imperceptible to the human eye, moving at several thousand kilometers per hour. This is just for plain glass plates as well and those cracks can propagate even faster if you mold the glass in a special way.
A Prince Rupert’s Drop is an amazing example of this. They’re a curious thing to behold, resembling a tadpole, and are created by dropping molten glass into a bucket of water. The bulbous end is surprisingly durable, able to withstand punishment that’d shatter any other type of glass (like hitting it with a hammer) but if the wispy tail sustains even the slightest dent the entire structure will rupture violently. So much energy is released in the reaction that the fracture actually propagates at 1.9km/s which you can witness in gorgeous slow motion here.
I’ve often though of getting a couple to keep around the house as a conversation piece but I’d fear they wouldn’t last the weekend with me wanting to see them explode.