A lens is a fundamental transmissive optical device made of transparent materials such as optical glass and high-purity plastic, which manipulates light propagation through the physical phenomenon of refraction. Derived from the Latin word “lens” meaning lentil, most traditional lenses feature spherical curved surfaces, and their core function is to converge or diverge light rays, laying the foundation for almost all optical imaging equipment. Unlike flat transparent glass that only allows light to pass through linearly, lenses change the direction of light travel by utilizing the difference in refractive index between transparent materials and air, enabling precise control of light beams.

Lenses are mainly divided into two core categories: converging lenses and diverging lenses, each with distinct structural characteristics and optical properties. Converging lenses, also known as convex lenses, are thicker at the center and thinner at the edges, including biconvex, plano-convex, and meniscus convex types. When parallel light rays pass through a convex lens, the light refracts inward and converges at a fixed point called the focal point. The distance from the lens center to the focal point is defined as the focal length, a key parameter that determines the lens’s optical power. The shorter the focal length, the stronger the ability of the lens to gather light.

In contrast, diverging lenses, or concave lenses, are thinner at the center and thicker at the edges, covering biconcave, plano-concave, and meniscus concave structures. Parallel light passing through a concave lens will refract outward and scatter in all directions. Human eyes cannot perceive the divergence of light directly, so these scattered light rays seem to extend backward to form a virtual focal point on the incident side of the lens. Another important concept in lens optics is the thin lens, which refers to a lens with extremely small thickness relative to its focal length. For thin lenses, we can ignore the slight deviation of light refraction inside the lens and simplify optical calculation rules, which is widely used in basic physical optics research and daily optical design.

The imaging rules of lenses follow strict physical laws. Convex lenses can form diverse images with different sizes and properties according to the distance between objects and the lens, including inverted real images and upright virtual images. Concave lenses can only form upright, reduced virtual images regardless of object distance. These basic optical principles are not only the core content of physical optics but also the essential theoretical basis for the design and manufacture of all optical instruments, making lenses an indispensable basic component in modern optical technology.