Lens (optics) concave surfaces is "biconcave" (or just "concave"). If one of the surfaces is flat, the lens is "plano-convex" or "plano-concave" depending on the curvature of the other surface. A lens with one convex and one concave side is "convex-concave" or "meniscus". It is this type of lens that is most commonly used in corrective lenses. If the lens is biconvex or plano-convex, a collimated beam of light passing through the lens converges to a spot (a "focus") behind the lens. In this case, the lens is called a "positive" or "converging" lens. The distance from the lens to the spot
Compound refractive lens the order of meters, by using many lenses in series. To minimize the absorption the lens must be made from a material such as aluminium, beryllium, or lithium. CRLs were first demonstrated in the mid-1990s by a group of scientists at the ESRF. They drilled holes in an aluminium block, and achieved focusing in two dimensions. For X-rays a concave lens focuses the X-rays because the index of refraction is slightly below unity. In a CRL of this type the walls between the cylindrical holes act as concave lenses for X-rays traveling perpendicular to the axis of the drilled cylinders.