The term double concave refers to a shape or surface that has two inward curves. Synonyms for double concave include bi-concave, biconcave, and bi-convex. These terms are often used interchangeably when describing lenses or mirrors with curved surfaces. Another synonymous term is negative lens, which describes a lens that causes light to diverge. Double concave lenses are used in a variety of optical devices, including telescopes and microscopes, to focus and redirect light. They are also commonly used in eye glasses to correct vision problems. Regardless of the term used, double concave surfaces are essential components in many important technologies.