Lens

A lens is an optical element that redirects light by refraction, reflection, or both. Its shape and material determine whether it converges or diverges light.

Refraction: The change in direction of a wave as it passes between materials with different propagation speeds. Refractive lenses redirect light because its speed changes at each material boundary.
Convex lens: A lens thicker at its center than at its edges, usually converging parallel light in air. It is the common positive-power form that brings rays toward a focus.
Eyeglasses: Wearable optical devices that use lenses to correct refractive errors or protect the eyes. Their prescriptions use lens power to compensate for focusing errors in the eye.
Mirror: An optical surface that redirects light primarily by reflection. Mirrors focus or redirect light without relying on transmission through a refracting body.
Nimrud lens: A polished rock-crystal object found at Nimrud, often discussed as an early possible optical lens. It illustrates how difficult it is to establish the purpose of ancient lens-shaped objects.
Focal length: The distance from an optical system’s principal plane to its focal point for parallel incoming rays. It quantifies a lens’s converging or diverging power.
Concave lens: A lens thinner at its center than at its edges, usually diverging parallel light in air. It is the common negative-power form that spreads rays apart.
Camera lens: An optical assembly that focuses light to form an image on a camera sensor or film. It combines lens elements to control focus, field of view, and image quality.
Prism: A transparent optical element with flat faces that bends and can disperse light by refraction. Prisms redirect or separate light, but typically do not form images by lens-like curvature.
Reading stone: A magnifying lens placed over text to make letters appear larger. Early reading stones applied lens magnification before spectacles became widespread.

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