Biconvex lenses have two outward facing convex curves producing positive focal lengths. They bend parallel rays so that they converge to a focal point. Singlet biconvex lenses are the second most simplistic lens design after plan convex lenses so have limitations in producing quality images. Biconvex lenses are used for simple focusing and light collecting applications. They are normally made from medium index crown glass or high index material for short F#. For UV applications UV fused Silica is used to extend workable range to 175nm.