invisible ray
Technical term used in physics and optics for electromagnetic radiation outside the visible spectrum, such as ultraviolet or infrared rays.
紫外線は不可視光線の一種です。
Ultraviolet rays are a type of invisible ray.
Compound of 不可視 (invisible) and 光線 (light ray). The exact historical derivation is uncertain, but the term follows standard Sino-Japanese word formation patterns.