Biconvex

A biconvex lens, also called a double-convex lens, is a lens with two outward-curved surfaces. Light entering from either side is refracted toward the lens axis and converges to a focal point, giving it a positive optical power. When the curvatures of the two surfaces are equal, the lens is symmetric; asymmetric variants adjust focal length and aberrations by using different radii on each side.

Optical properties and behavior: The focal length of a biconvex lens depends on the material’s refractive index and the curvatures of both surfaces. In thin-lens approximation, the focal length decreases as the curvature becomes stronger (smaller radii) or the refractive index increases, increasing the lens’s converging power. Biconvex lenses are commonly chosen to produce a positive focal length in compact form and are used when symmetric focusing is advantageous. In practice, design trade-offs with spherical aberration and chromatic effects lead to a range of curvatures and materials to suit specific applications.

Materials and manufacturing: Biconvex lenses are produced in glass (such as crown or flint varieties) or transparent plastics (including PMMA and polycarbonate). Glass lenses are typically ground and polished to precise radii, while plastic lenses may be molded. Anti-reflective coatings and subtle aspheric corrections may be applied to improve transmission and reduce aberrations.

Applications and context: They are used in eyeglasses for farsighted correction, camera and projector lenses, microscopes, and various scientific and optical instruments. Compared with plano- or concave-convex designs, biconvex lenses provide positive focusing power in a relatively compact form, though they may be heavier and thicker at the center for higher powers.