● To collect a lot of light from a light source, a single lens needs to have:
               -- a large diameter
               -- a short focal length
                         -- implies a large, steeply-curved lens
                                        (―)  thick, heavy, take up a lot of space, absorb a lot of light, often crack

     ● In 1822 Augustin Fresnel conceived of a way to reduce the thickness, yet maintain the refracting power of a lens:
               He recognized that the refraction occurs at the surface, not in the thick middle part of the lens.
               He cut the lens in sections, and removed most of the glass in the middle, leaving "rings" of glass.
               When collapsed onto a flat back surface, a thin lens resulted that had the same focal length as the thick lens.

     ● Frenel's first application was to make lenses for lighthouses.

     ● Used in lighthouses, spotlights, theater lighting, overhead projectors, camera viewfinders, traffic lights, lanterns.

     ● "Take our textbook's discussion of 'masking' a traffic light to project light into only certain lanes...........
                       ............... with a grain of salt."
  Animation of how a Fresnel lens works.