A convex lens causes light that falls on it to pass through a single point called the focus or focal point when the light is parallel to a line drawn through the centre of the lens, known as the principal axis. Because light can travel the same way in either direction, light coming from the focus is concentrated into a parallel beam coming out of the lens. (Thus, it works in a similar way to the parabolic metal reflector.)
The use of a prism in an optic is rather more complicated because, depending upon how it is used, will depend upon what the result is. For example, a prism used in the way below will simply cause refraction.
Of much more importance is the situation like that shown below where the ray of light is reflected in the prism. Much greater changes in the path of the light ray can be achieved like this. So why is this more important? In a practical sense there are limits on the size of lens that you can use. If you make it bigger and bigger you always reach a limit where a larger size does not achieve the results you want because the lens is simply not capable of re-directing the light and maintaining the beam that you are trying to create. By using prisms around the lenses, you can collect more light and make the beam even brighter. you will see this in the last diagram below.
Such a panel is shown in the final image below. In this photo, you can see two similar panels, side by side. Each has a central lens with arcs of prisms surrounding the bull's eye to capture as much of the light from the source as possible. Those prisms close in to the bull#s eye probably refract the light, keeping it in a beam with the rays passing through the bull's eye. The outer prisms will reflect the light using the method above, again to keep all rays in line with the main beam. This is a Fresnel-type optic, named after the French designer who invented the principle.
