How to Build a Laser Death Ray
In brief: Ray beam weapons emit needle-thin beams that produce a white-hot plasma along their path and easily burn deep holes into their targets. They look and sound like bolts of straight lightning.
Detailed description: There are a wide variety of emmanations that can be broadly classed as "light", both invisible and visible. Lasers can be made to emit any of these. Those which are shorter wavelength, higher frequency, and higher energy per particle than the familiar visible light interact differently with matter than the more typical near ultraviolet, visible, and infrared electromagnetic emissions. The light in the vacuum ultraviolet, extreme ultraviolet, and soft x-ray region of the spectrum has unique properties that render death rays that use pulses of these wavelength significantly different from the effects of near visible blasters.
Air is transparent to the light of blasters, but the plasma the blaster pulse creates when incident upon condensed matter is opaque to their light. As a result, a thin layer of plasma absorbs all the light and then violently explodes. Ray beams have the opposite issue. They easily penetrate the energetic plasmas they create, allowing the beam to pass through and drill deeply. However, non-ionized matter of all kinds is essentially opaque to these radiations - including air. Consequently, the ray beam has to "burn" through air, ionizing it to a plasma state in order for the beam to penetrate.
A consequence of this is that the beam needs to be very narrow in order to avoid wasting a lot of energy burning through air between the laser and the target. However, as you will learn in the section on Diffraction, the narrower the beam, the faster it spreads out. Diffraction is less of an issue for these very short wavelength radiations, but it is still important for sub-millimeter wide beams if you want them to maintain focus over distances of tens or hundreds of meters.
In vacuum, these problems go away and the beam can be emitted from a wide aperture and focused to a point, just as for any heat ray or blaster. Because of their short wavelengths, ray beams can be focused more tightly from a given size aperture at any given range than near visible varieties of light. Thus if ray beams are technologically possible they are likely to be preferred for combat in space.
At extremely short wavelengths, when you get to the hard x-ray and gamma ray parts of the spectrum, radiation can again go some distance through air and even solid matter. These beams of penetrating ionizing radiation start to behave differently from the class of ray beams described here, but because of radiation health issues they are unlikely to be used by science fiction heroes except from within radiation shielded armored fighting vehicles.