Here’s a really cool paper from two groups in South Africa. It involves shooting a laser through fire! How awesome can you get. And the tests that they came up with to demonstrate that it was working are also really cool and a bit funny. Read on!
The principle of the experiment is that a a flame can focus light passing through it — this is long known, and is because of the varying density of gas through the flame causes the laser to refract, just like the different thickness of glass in a regular lens causes the light to focus.
A great thing about using gases and flames as lenses is that they can’t be damaged by intense lasers in the same way that glass or other materials might be.
However, up to now, most flame lenses couldn’t produce a nice, sharply focused dot.
The idea implemented by the groups at the University of KwaZulu–Natal and the National Laser Centre in South Africa, was to combine two lens elements – a cylindrical lens that acts mostly on the outermost portion of the laser light, and a spiral lens (in which the flame from an oxy-acetelyene torch is forced around in a spiral — how cool is that?) to focus the inner rays.
If the flames themselves weren’t awesome enough, the diagnostics are even better. The first compares shooting a laser onto a piece of plastic with the lens on and off. In the authors’ words:
“With the flame turned off, the laser beam makes a barely perceptible sound as each unfocused pulse illuminates the plastic. The moment the flame is ignited loud ‘cracks’ are audible and a bright plasma plume appears.”
Another diagnostic is where they make a map of Africa with LEDs — this starts out blurry, but once the flames are ignited, a much shaper image is produced.
What a great and entertaining paper! Must have been lots of fun to work in this lab!
Max M. Michaelis, Cosmas Mafusire, Jan-Hendrik Grobler, Andrew Forbes (2013). Focusing light with a flame lens Nature Communications DOI: 10.1038/ncomms2894