How to Build a Laser Death Ray
Far Infrared Light
Far infrared is the color of light emitted as thermal radiation by room temperature objects. It is what is being seen by thermographic imagers, that detect people by their body heat. Air is fairly transparent to far infrared light in the "window" from 8 to 12 microns.|
However, far IR cannot penetrate water more than a few tens of microns. In fact, most stuff that is transparent to visible light is opaque to far infrared. It will not go through glass, plastic, or quartz. Only a few materials are suitable for making windows or lenses for light from a far IR death ray. A single crystal of table salt is the most redily available, but this has problems with absorbing water and dissolving. Other far IR-transparent materials tend to be fairly pricey.
Far infrared has a long enough wavelength that it has definite issues with focusing (due to diffraction) and, at high intensities in air, cascade ionization. It is best avoided in favor of shorter wavelengths if possible. One reason it might be used is for a death-ray-on-a-budget. The common and fairly efficient carbon dioxide lasers emit light in the far infrared (at 10.6 microns). A high powered CO2 laser could make an effective heat ray at short to moderate ranges.