How to Build a Laser Death Ray
Visible light is, of course, the light that we can see. Pure frequency visible light includes
Air is highly transparent to visible light.
If the expense of overcoming atmospheric twinkle is not too much of a burden, visible light is ideal for use in an atmosphere due to its long attenuation length and sharp focusing power. Depending on the amount of scattering, a visible light death ray beam may be obvious to the naked eye, especially indoors or at night. When the beam strikes matter, some of the beam will be scattered, creating a dazzling flash or flare.
Not many lasers can produce the high powered beams of visible light needed for use as a weapon. Free electron lassers could, but none are being designed to work at high powers in this wavelength range. There is a trick to get near infrared lasers to produce visible beams, however. Certain crystals can double the frequency of light that goes through them. This can allow Nd:YAG lasers to have their beams up-converted to visible green, for example. As far as this author knows, however, no visible light death rays are under consideration as weapons at this time. There is no reason, however, that they cannot be used in science fiction.