lens

A lens is a piece of optically transparent material, typically glass or plastic, shaped so that its surfaces bend (refract) light rays in a controlled way. The two surfaces are usually spherical, though they may be flat, cylindrical, or aspheric. By changing the path of light, a lens either converges rays toward a focal point or spreads them apart, making it fundamental to cameras, telescopes, microscopes, projectors, and eyeglasses.

The most common lens types are convex (thicker in the middle) and concave (thinner in the middle). A convex lens converges light and produces a real, inverted image when an object sits beyond its focal length; it acts as a magnifying glass when the object is closer. A concave lens diverges light and always produces a virtual, upright, diminished image. Most optical systems use multiple lenses combined to correct aberrations and achieve the desired magnification, focal length, or field of view.

Materials and coatings

Crown glass, with a refractive index around 1.52, remains the standard for general-purpose lenses. Flint glass, with indices from 1.6 to 1.75, provides higher dispersion and is used in precision applications to reduce chromatic aberration. Plastic lenses (polycarbonate, acrylic) are lighter and more impact-resistant but suffer more scratching and thermal distortion. Anti-reflection coatings, typically single or multi-layer interference films, reduce surface reflections from roughly 4 percent per air-glass interface down to below 1 percent, critical in high-performance systems.

Optical defects limit every lens. Spherical aberration occurs because spherical surfaces do not focus all rays to the same point; parabolic or aspheric profiles reduce this but are expensive to manufacture. Chromatic aberration splits white light into spectral colors because refractive index varies with wavelength; it is corrected by pairing lenses of different glasses. Distortion bows straight lines; vignetting darkens the image edges. Dust, scratches, and internal strain birefringence all degrade image quality.

The term lens derives from the Latin for lentil, whose seed resembles the biconvex shape of an optical lens. Optical design relies on the lensmaker's equation, which relates focal length to the refractive indices and radii of curvature. In practice, lens performance is specified by focal length (distance from the lens where parallel rays converge), f-number (the ratio of focal length to the diameter of the aperture), and numerical aperture (which defines the light-gathering power and resolution limit in microscopy).