What is defined as having one convex and one concave surface?

A meniscus lens is characterized by having one convex surface and one concave surface. This unique shape allows the lens to focus light more effectively than a simple lens that has uniform curvature on both sides. The convex side bulges outward, helping to converge light rays, while the concave side curves inward, which can assist in diverging or managing the light passing through the lens. The specific combination of these two surfaces enables the meniscus lens to provide various optical properties based on its curvature and the material used.

In contrast, an aspheric lens typically has a complex surface profile that does not conform to the simple spherical shape, making it suitable for reducing spherical aberration in high-performance applications. A bifocal lens contains two distinct optical powers rather than different curvatures on its surfaces. A flat lens has no curvature and generally produces minimal optical distortion, lacking the distinctive convex and concave characteristics of a meniscus lens. Understanding these differences illustrates why the meniscus lens is the correct identification for a lens with one convex and one concave surface.