An Etymological Dictionary of Astronomy and Astrophysics

The → electromagnetic radiation divided into its constituting wavelengths or frequencies.

Etymology (EN): From L. spectrum “appearance, image, apparition,” from specere “to look at, view;” Gk. skopein “to behold, look, consider,”
skeptesthai “to look at;” PIE base *spek- “to see;” cf. Av. spas-, spaš- “to attend to; to serve; spy,” spasiieiti “looks at, perceives” (Mod.Pers. sepâs “kindness, favor, thanksgiving,” sepâsidan “to praise for benefits received);” Skt. paś- “to see, watch,” spasati “sees;” L. specere “to look at;” O.H.G. spehônn “to spy,” Ger. spähen “to spy.”

Etymology (PE): Binâb “a vision;” Mid.Pers. wênâb “vision,” from wên-, present stem of didan “to see;”
O.Pers. vain- “to see;” Av. vaēn- “to see;”
cf. Skt. veda “I know;” Gk. oida “I know,” idein “to see;” L. videre “to see;” PIE base *weid- “to know, to see.”