Telescopes

A telescope makes far-off things look nearer and shows faint ones the eye can't see. A refracting telescope has two converging lenses: a big objective lens with a long focal length makes a small real image of a distant object, and an eyepiece with a short focal length magnifies it like a magnifying glass. A reflecting telescope, Newton's idea, uses a big concave mirror instead of the objective lens; all the largest telescopes are reflectors, since a mirror can be supported from behind. The wider a telescope's opening, the more light it gathers, so the fainter the things it shows.