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Lasers  [ Popular misconceptions ]

The representation of lasers in popular culture, especially in science fiction and action movies, is often misleading. Contrary to their portrayal in many science fiction movies, a laser beam would not be visible (at least to the naked eye) in the near vacuum of space as there would be insufficient matter off of which to scatter.

In air, however, moderate intensity (greater than ~10 milliwatts) laser beams of shorter green and blue wavelengths and high intensity beams of longer orange and red wavelengths can be visible due to Rayleigh scattering. With even higher intensity pulsed beams, the air can be heated to the point where it becomes a plasma, which would also be visible. This would also cause a rapid heating and explosive expansion of the surrounding air which would produce a popping noise analogous to the thunder which accompanies lightning. This phenomenon is also capable of causing a retroreflection of the laser beam back into the laser source possibly damaging its optics. When this phenomenon occurs in certain scientific experiments it is referred to as a "plasma mirror" or "plasma shutter".

Science fiction films special effects often depict laser beams propagating at only a few metres per second—slowly enough to see their progress, in a manner reminiscent of conventional tracer ammunition—whereas in reality a laser beam travels at the speed of light and would seem to appear instantly to the naked eye from start to end.

Some action movies depict security systems using lasers of visible light (and their foiling by the hero, typically using mirrors); the hero may see the path of the beam by sprinkling some dust in the air. It is actually far easier and cheaper to build infrared laser diodes rather than visible light laser diodes and such systems almost never use visible light lasers.

Several of these misconceptions can be found in the James Bond film Goldfinger, the first film to feature a laser. In one of the most famous scenes in the Bond films, Bond, played by Sean Connery, faces a laser beam approaching his groin while melting the solid gold table to which he is strapped. The director Guy Hamilton found that a real laser beam would not show up on camera so it was added as an optical effect. The melting effect on the table was achieved by a man underneath the table holding an oxyacetylene torch, while a real laser would have produced a fairly heat-free and silent cut.

In addition to movies and popular culture, laser misconceptions are present in some popular science publications or simple introductory explanations. For example, laser light is not perfectly parallel as is sometimes claimed; all laser beams spread out to some degree as they propagate due to diffraction. In addition, no laser is perfectly monochromatic (i.e. coherent); most operate at several closely spaced frequencies (colors) and even those that nominally operate a single frequency still exhibit some variation in frequency. Furthermore, mode locked lasers are designed to operate with thousands or millions of frequencies locked together to form a short pulse.


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