Afocal

Part of speech: adjective

Definitions

1. Pertaining to an optical arrangement in which light rays remain nearly parallel and do not converge or diverge significantly
2. Relating to a system characterized by the absence of a defined focal point, allowing for images to be clear at infinite distances
3. Describing a configuration that lacks a focal point, resulting in light rays traveling parallel without focusing or spreading out significantly

Etymology: "Afocal" is a plain technical formation: the prefix "a-", meaning "without" (from Greek "a-", "not" or "without"), attached to "focal", the adjective from "focus." Together they describe something lacking a focus, most often in optics. An afocal system is one whose lenses or mirrors send parallel light rays out as parallel rays, so it has no single point where the light converges. The "focal" half comes from Latin "focus," which meant "hearth" or "fireplace" and later came to mean the point where rays meet. Johannes Kepler adopted the Latin word in the early 1600s for that convergence point, and "focal" followed as its adjective. Attaching the Greek-derived "a-" to it gave optics a precise way to name the absence of that point.