It's a balmy summer day, and you're cooling off with a frosty beer. To you, your drink looks like, well…a drink. But all around it, invisible to the naked eye, cold vapors are sinking into space, creating ripples of bubbling air. It would look pretty cool, if only you could see it.
This is called the "Schlieren effect" or "Schlieren flow," and it's based on the principle of refraction. When light passes through air of different densities, it fluctuates and bends in a particular way. The end result is pretty trippy, and according to the YouTube channel brusspup, it's also easy to recreate in a simple household experiment.
First, you're going to need a few gadgets. Mainly, a parabolic mirror, camera, point light source, and some type of sharp edge, like a razor blade. If you don't have any of these things, don't fret, because this video does it all for you.
As you can see, objects that generate heat produce "bubbles" that seem to rise upward in front of the mirror. Whereas cold objects, such as a popsicle, produce descending vapors. The forces at play are best described by Bryan Rolfe, who explains that "when light passes through air of higher density, it refracts and passes to one side of our focal point. Likewise, light passing through less dense air will bend slightly in the other direction, and pass to the other side of our focal point."
The crux of this experiment is the razor blade, which helps to create an optical path of focused light. According to Rolfe, "light that's refracted through the less dense air gets rejected, and light that's refracted through the denser air gets passed." This trick reveals pockets of air that would otherwise be invisible.
The Schlieren effect earned its name from the German word "schliere," which means "streak." In 1665, the curious phenomenon was first described by the English philosopher, Robert Hooke, who observed the flow of hot air produced by a candle flame. More recently, the principle has been used in certain video projectors, though this method has been largely replaced by the LCD projector.