Section of 36 inch high fixed-lens (fourth order) lighthouse optic, 1890-1900.
© Science Museum / Science & Society Picture Library
Large lenses, like those used in lighthouses, can be very heavy and distort at the edges. In 1748 Georges de Buffon thought of a way to lighten them and reduce energy los. He realised that only the surface of a lens is needed to bend light, because once the light is inside the lens it travels in straight lines. The insides of the lens could be cut away, leaving rings with angled edges. Buffon's idea was modified by Augustin Fresnel (1788-1827), who fitted lenses in lighthouses.