LENSES (refraction of light)

A lens can be thought of a series of prisms which combine to bring the light to a single point or "to focus".  Thus, lenses can be classified as one of two types: Convergent and Divergent:

A Converging Lens bends Incident Rays from infinity so that they intersect at a common point.
A Divergent Lens bends Incident Rays from infinity backwards so that they intersect at a common point. (in this lens, the glass is concave instead of convex)

We call the point at which the rays intersect the "Point of Focus"