Many solar collectors (or reflectors) contain a parabola. A parabola has the property that light approaching the parabolic curve in a direction parallel to the axis is reflected back through the focal point. Thus, light rays from the sun (which are almost parallel) are reflected back from the solar reflector, and concentrated at a point on the (illustrated) ‘focal line’. There they heat up water contained in a pipe which is then used for heating or power generation.
