A transparent optical device that changes the directional properties of a beam of light passing through it. Different types of lens are designed for different purposes. Convex and biconvex lenses cause a parallel beam of light to converge at the focal point. Concave and biconcave lenses diverge a parallel beam. Lenses are frequently used in combination to achieve results unobtainable with a single lens, for example in a telescope eyepiece. Elements may be cemented together: doublets and triplets are lenses composed of two and three elements, respectively. Such combinations, made from lenses of different types of glass, are used to minimize chromatic aberration. The objective lens of a refracting telescope is its light-gathering element.