Which of the following lenses is commonly used to focus light?

Biconvex lenses are often used to focus light due to their shape, which is thicker at the center and thinner at the edges. This design causes parallel rays of light that enter the lens to converge, focusing them to a point on the opposite side of the lens known as the focal point. This type of lens is essential in various optical devices, such as magnifying glasses, microscopes, and cameras, where precise focusing of light is required.

Biconcave lenses, on the other hand, are designed to diverge light rays rather than converge them, which makes them suitable for different applications, such as expanding a beam of light. Plano-concave lenses, similar in function to biconcave lenses, also diverge light due to their shape. The term "convex" is often general and could refer to any lens that bulges outward, but in the context of specific lens types, the biconvex lens is the definitive choice for focusing light, as it is explicitly designed for this purpose.