- Line of pale dust thrown up by meteorite impact, observed on the surface of the Moon and Mercury, leading back to the crater whose creation produced the ray. Ray craters are thought to be the youngest on the Moon, some as recent as a few hundred million years.
- A line of radiant energy, such as light, whose cross section is very small. Also, a representation of such a line. For example, the path of a single photon.
- A particle or beam of particles emitted or radiated from a source. For instance, alpha rays, beta rays, cathode rays, or gamma rays.
- A particle or beam of particles whose cross section is small or very small. For example, that utilized in electron beam lithography.
- One of multiple lines diverging from a single point. For example, radii from a circle. Also called half-line.
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