After 101 days of traveling by plane, train, automobile, Korean warship, zipline and even robot, the Olympic torch will finally reach the site of the Winter Games in PyeongChang, South Korea.
Last Friday, a lucky honoree will use it to light the Olympic cauldron in a grand, symbolic start to the games.
While the blaze looks like any other, its origins are special: It was lit not with matches or a Zippo lighter, but with a parabolic mirror, echoing rituals from Ancient Greece.
To brush up on algebra, a parabola is a particular type of arc that is defined by the exact curvature of its sides.
Mathematically, these symmetrical curves all take some form of the equation, Y = X^2. Revolve a parabola around its axis, and you have the shape of a parabolic mirror.
Unlike most curves, which scatter incoming light in many directions, the reflected beams bounce from a parabola and all concentrate to one point, the focus.
These reflective surfaces are used in a number of devices to concentrate not only reflected light, but also sound or radio waves.
Satellite dishes, some types of microphones, reflecting telescopes and even car headlights benefit from the reflective properties of parabolic dishes.
In the case of the Olympics, when the sun shines on a parabolic dish, known to the ancient Greeks as a Skaphia or crucible, the rays all bounce off its sides and collect at one blazing hot point.
Put a piece of paper—or a gas torch—in that focal point, and you get fire.
A lone parabolic dish does a decent job heating things up, achieving temperatures of at least hundreds of degrees.
“That’s really very easy to reach,” says Jeffrey Gordon, professor of physics at Ben-Gurion University of the Negev in Israel.
Some may even be able to reach temperatures in the thousands of degrees, says Jonathan Hare, a British physicist and science communicator.
Hare has witnessed parabolic mirrors vaporize carbon, something that only happens at temps over 2,000 degrees Celsius
If conditions are absolutely ideal, light can be concentrated to match the same temperature as its source, Gordon explains. In the case of the sun, that means that the upper temperature limit when concentrating its rays is around 10,000 degrees Fahrenheit.
“No matter what you do, no matter how brilliant you are, you can never bring any object on Earth to a higher temperature [by concentrating sunlight],” says Gordon.
But, of course, conditions are never ideal. First, some of that heat is lost to the atmosphere.
Then, some is absorbed into your reflective surface, and still another fraction is scattered away due to imperfections in the mirror.
“The parabola is a good concentrator but not a perfect concentrator,” Gordon adds.
Gordon’s research is focused on pushing the limits of sun concentration to the max.
Using multiple concentrating mirrors, his lab has achieved temperatures of nearly 3,000 degrees Celsius, applying the heat for a range of feats, including a sun-powered surgical laser and a reactor for creating nanomaterials.
But now, at some truly blistering temps, he has a different problem.
“We start to destroy everything,” he says.
In the case of Olympic torch lighting, the issues are somewhat more mundane. For one, there’s the potential for clouds.
In the days leading up to the modern torch lighting ceremony at the ancient temple of Hera in Olympia, the organizers light a flame in a parabolic dish, just in case clouds obscure the sun on the day of the ceremony.
The preparedness proved useful at this year’s event, which took place on the drizzly morning of October 24, 2017.
People have practiced the concentration of the sun’s rays for thousands of years. The most famous example of solar concentration comes from 212 B.C. during the siege of Syracuse, Greece.
The Greek mathematician and inventor Archimedes used the parabolic mirror, so the story goes, to deter a fleet of approaching ships, crafting a solar death ray using panels of what was likely polished bronze.
Though there’s reason to doubt the veracity of these somewhat fantastical claims—including a failed MythBusters’ attempt to replicate the feat—the ancient Greeks did have a handle on the magic of these special curves.
The first torches used in the games were modeled after ancient designs, writes Chapoutot. Built by the Krupp Company, Germany’s largest armament producer, each one only burned for 10 minutes.
The torches used today have come a long way.
In recent years, organizers have opted for high-tech features to keep the flame lit, no matter the weather.
This year’s torch, dreamed up by Korean designer Young Se Kim, has four separate walls to ensure the flame can withstand winds up to 78 mph.
It also has a tri-layered umbrella-like cover to prevent rain from extinguishing the blaze. It can even withstand temperatures down to -22 degrees Fahrenheit thanks to its internal circulation system.
If the flame goes out en route, support is always nearby with backup fire lit by parabolic mirror to swiftly relight it. Though the flame has averted major disasters this year, its robot transporter almost tipped over.
Organizers rushed to right the bot, preserving the flame.
So during last friday’s opening ceremony, as the Olympic cauldron is lit, take a moment to appreciate the fire that roared to life under a glowing bath of concentrated rays of sunlight.
As Greek archaeologist Alexander Philadelphus described during the planning of the first torch relay, the warm glow wasn’t lit by modern mechanics, but rather came directly from Apollo, “the god of light himself.”
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