A lens is a specially shaped pieces of glass or transparent plastic, that is used to focus light. A convex lens is made from a transparent material that bulges outwards in the middle on both sides.

A spyglass with a plano-convex lens that focuses at 12 inches and a ... cheered the coming day when aerial telescopes would have a focus of 1,000 feet and human spectators could marvel at the ...

An optical lens is a transparent optical component used to converge or diverge light emitted from a peripheral object. The transmitted light rays then form a real or virtual image of the object.

This is because the eye refracts the light rays and they are brought to a focus having passed the retina. This is corrected by using a convex or converging lens.