Newton chose 'spectrum' (Latin for 'phantom') in 1671 — the rainbow seemed an apparition revealed by glass.
A band of colours produced by the separation of light; a range or scale between two extremes; in physics, the distribution of a property across a range of values.
From Proto-Indo-European *speḱ- (to observe, to look at), which yielded Latin specere (to look). From specere came spectrum (an appearance, image, apparition), the direct ancestor of the English word. The PIE root *speḱ- is extraordinarily productive: it generated Latin species (appearance, kind), speculum (mirror), spectare (to watch), and through these gave English spectre, species, especial, spy, and inspect. Isaac Newton borrowed
Isaac Newton coined the optical sense of 'spectrum' in 1671, choosing a Latin word that meant 'phantom' or 'apparition.' He saw the rainbow of colours emerging from a prism as a kind of ghost — a hidden reality made visible. The related word 'spectre' (a ghost) comes from the same root, and in some
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