Lenses are materials that are curved in order to change the way light passes through. Lenses occur in nature and technology.

A selection of different types of lenses. image: wikipedia

Within optical and visual technology, lenses are optical elements designed to bend light usually to either focus light to a focal point or disperse light or some variation between the two. Combinations of lenses are used to achieve specific goals in dealing with often specific wavelengths of light. 


Concave lenses tend to create divergent(spread out) beams of light. image: wikipedia


Convex lenses tend to converge or focus light beams to a point. image: wikipedia

Using combinations of lenses can be used to 'bring colours' together. This is useful in microscopes and zoom lenses to avoid a rainbow effect around viewed objects. image : wikipedia

see also: http://en.wikipedia.org/wiki/Lens_(optics)