Lens

A lens is a transmissive optical device. It works by focusing or dispersing a light beam through refraction. Refraction is the bending of light as it passes through a material. A single piece of transparent material is a simple lens. When several simple lenses, called elements, are arranged along a common axis, they form a compound lens. Lenses are typically made from glass or plastic. They are manufactured by being ground, polished, or molded into specific shapes.

The way a lens functions depends on its geometry and the material used. Most lenses are spherical lenses. This means their surfaces are parts of spheres. Each surface can be convex, which bulges outward. It can be concave, which is depressed inward. It can also be planar, or flat. The line connecting the centers of these spheres is the lens axis.

Lenses are classified by the curvature of their two optical surfaces. A biconvex lens has two convex surfaces. If these surfaces have the same radius of curvature, it is an equiconvex lens. A biconcave lens has two concave surfaces. If one surface is flat, the lens is called plano-convex or plano-concave. A meniscus lens has one convex side and one concave side.

These shapes determine how light behaves. A biconvex or plano-convex lens in a lower-index medium is a converging lens. It causes a collimated beam of light to converge to a single spot called a focus. The distance from the lens to this spot is the focal length. Conversely, a biconcave or plano-concave lens is a diverging lens. It causes light to spread out, appearing to emanate from a point in front of the lens.

Humanity has interacted with lenses for millennia. The word "lens" comes from the Latin name for a lentil seed. This is because a double-convex lens resembles the shape of a lentil. Archaeological evidence suggests widespread use in antiquity. The Nimrud lens is a rock crystal artifact from the 7th century BCE. It may have served as a magnifying glass or a burning glass. Pliny the Elder confirmed that burning-glasses were known during the Roman period in the 1st century.

During the Middle Ages, scholars like Ptolemy, Ibn Sahl, and Alhazen studied optics. Between the 11th and 13th centuries, people used "reading stones." These were primitive plano-convex lenses made by cutting glass spheres in half. Spectacles were later invented in Northern Italy during the second half of the 13th century. This led to the optical industry of grinding and polishing lenses. This progress eventually resulted in the compound microscope around 1595 and the refracting telescope in 1608.

Technological needs drove further innovation. In 1733, Chester Moore Hall invented the compound achromatic lens in England. This helped correct chromatic errors, where different colors fail to focus at the same point. In 1823, the Fresnel lens was first implemented in a lighthouse. These lenses used concentric annular sectioning to use less material. This made them practical for the massive sizes required for maritime navigation.

Lenses are essential to modern science and daily life. They are used in telescopes, binoculars, and cameras. They also serve as visual aids in glasses to correct vision defects like myopia and hypermetropia. Beyond visible light, the term "lens" applies to other waves. We use microwave lenses, electron lenses, and acoustic lenses. These tools allow us to study everything from the smallest particles to the vastness of space.