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Optical physics

Optical physics

Edible lasers and death rays

17 Apr 2014
Taken from the April 2014 issue of Physics World

How the Ray Gun Got its Zap: Odd Excursions Into Optics
Stephen R Wilk
2013 Oxford University Press £18.99/$34.95hb 256pp

Playing with light

“Optics is light work” was one of Arthur Schawlow’s favourite slogans, and it didn’t just appear on the T-shirt he wore while giving lectures at Stanford University. Schawlow, who shared the 1981 Nobel Prize for Physics for developing laser spectroscopy, was a playful physicist who concocted novel experiments to amuse as well as educate. So it’s probably inevitable that there’s a Schawlow story in How the Ray Gun Got Its Zap, Stephen Wilk’s collection of “odd excursions into optics”.

The Schawlow story Wilk picks is a gem, though. In the decade after the first laser was demonstrated in 1960, so many different materials were made into lasers that Schawlow concluded “anything will lase if you hit it hard enough”. To prove this, he and Theodor Hänsch (then a young postdoc, later a Nobel laureate himself) decided to fire pulses of light at brightly coloured Jell-O brand gelatine desserts, in the hope of making an “edible laser”. None of the 12 flavours they bought in the supermarket would lase, but they eventually succeeded by adding a dye, sodium fluorescein, to unflavoured gelatine. Schawlow called the result “almost non-toxic”, but declined to eat the experiment afterwards.

After recounting this tale, Wilk, an optical engineer, goes in search of the truly edible “gin and tonic laser” he heard about as a doctoral student. Eventually, he traces the story to an experiment at Eastman Kodak labs, where two researchers found that very fast flashlamps could indeed excite blue laser pulses from an unidentified brand of tonic water. It didn’t make a good laser, Wilk observes, but it does make a fun story.

Wilk’s interest in such fun stories makes his book an entertaining tour of history’s optical oddities. The most intriguing mystery of ancient optics concerns Archimedes: did the great proto-physicist really mastermind the burning of a fleet of Roman ships by focusing sunlight onto them with polished shields? The surviving written evidence is scanty, and scientists and engineers have tried to resolve the question for centuries. A long list of experiments shows disconcertingly wide variations. The results ranged from modest heating to conflagration, often in keeping with what the experimenters hoped to find, so the question may be among the great unanswerables. All history has to say is that whether or not the Greeks burned some of their ships, the Romans ultimately won.

Many of the book’s essays answer odd questions raised by curious minds, such as why we think the Sun is yellow despite the fact that sunlight is, by definition, white light. That’s a puzzler to ponder, and Wilk points out flaws in three common explanations before concluding that the Sun looks yellow to the eye because the atmosphere scatters blue light across the sky. But that’s not quite the whole story, he adds, because when the Sun is high in the sky, too little blue light is scattered to make the Sun look yellow. The impression of a yellow Sun comes when it is low enough in the sky to glance at briefly, but not so low that it looks orange or red to the eye – colours that we know to be wrong.

Another, similar, essay concerns the number of colours in a rainbow. As a child, Wilk was told there were seven, but as he writes, “My old Crayola crayon box held 64 colours.” To resolve this paradox, he digs back to – what else? – a three-volume 1858 treatise on the Greek poet Homer. This work devoted a full 42 pages to Homer’s use of colour, and thereby launched an ongoing debate over how the ancients described it. The division of the spectrum into seven colours is sometimes linked to an attempt to mirror the seven notes of the musical scale, but Wilk traces it instead to a decision Isaac Newton made while writing his definitive treatise Opticks. In some places Newton listed only the colours red, yellow, green, blue and violet, but in others he added “orange” and “indigo”. Orange was soon ensconced as a definitive colour, but the distinction between blue and indigo is so subtle that indigo is often lost – except when an “i” is needed to make the colour mnemonic “Roy G Biv” pronounceable.

The book’s title comes from a wonderful chapter in which Wilk traces the history of fictional “death rays” back more than 200 years, to an 1809 novel in which the author Washington Irving – best known for The Legend of Sleepy Hollow – armed his interplanetary invaders with beams of concentrated sunlight. The “heat rays” of H G Wells’ better-known Martian invaders did not arrive until 1896. “Disintegrator rays” soon followed, and death rays of various types became standards of pulp-era science fiction, comics and films. In most cases, these rays killed on contact, leaving dead bodies but not the blood and guts of the deadly mechanized warfare that began with the First World War. Quoting the science-fiction critic Peter Nichols, Wilk notes that their invention “may have resulted from a certain squeamishness, since it allows for a maximum of destruction with a minimum of bleeding pieces to sweep up afterwards”. As a card-carrying laser and SF geek, I couldn’t ask for more.

How the Ray Gun Got Its Zap is not a big-picture, big-issue or deep-thought book. It’s an old-fashioned cabinet of wonders in book form, offered in the spirit of intellectual fun. It sent me down to the kitchen to see if my violet laser pointer would stimulate bright fluorescence from any of the leftover Christmas food colouring. The only glimmer of hope was from red cinnamon nonpareils, but I may put some coloured Jell-O on my grocery list.

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