Security Lasers

Quite often the drama in a crime movie or an adventure movie is enhanced by having crisscrossed visible laser beams around the item to be protected from theft. The thief must avoid intersecting these beams during the escapade to steal the item; otherwise a security alarm will notify the appropriate authorities and the thief will be caught. If you were the movie’s director, how would you make this scene to ensure good physics?

Answer

As the director of the theft scene, you would know that the laser light would not be visible in the air in a normal room because there is not enough scattering of the light to your eyes by the molecules in the air. Spots where the beams strike walls, mirrors, or any objects could be seen, but their straight paths to the spots are invisible. Therefore you have two ways to make the laser beams visible so your audience can see the sequence of maneuvers required to succeed in the heist: (1) put something in the air itself to scatter laser light such as chalk dust, smoke, or a fog of liquid nitrogen droplets or water mist, or (2) artificially put in the light beams during editing, after the scene is done.

We suppose that some real-world locations may use an arrangement of crisscrossed laser beams for security aids, but we do not know of any. Certainly, infrared lasers are much cheaper than visible lasers, and they would not be seen by injecting a fog into the space. Nor would their spots on the wall be seen.

Some heist scenes show the substitution of a mirror to fool the security system while the protagonist steals the item. Indeed, the mirror substitution may reflect the beam in the correct direction, but the tenths of a second disturbance in the original beam would be very easy to detect by the security system electronics, which can sense disturbance spikes quickly and sound an alert. Of course, the human security operator may choose to ignore the alert, which so often happens in movies!