In the centuries after the fall of the Roman Empire in the fifth century CE, the study of solar energy focused on so-called “burning glasses.” As noted by various Greek philosophers, mirrors and convex lenses can be used to concentrate the Sun’s rays, generate significant heat, and potentially start fires. Researchers throughout the Middle Ages made an effort to refine and improve burning glasses, and to experiment with mirrors to amplify their effect. In this regard, the centre of research shifted to the Muslim world in the tenth and eleventh centuries. Persian scientist Ibn Sahl investigated the properties of light and mirrors, and, in his work On Burning Mirrors and Lenses (984 CE), was the first to detail and explain refraction—the bending of light as it passes through different materials. Perhaps the most prolific researcher on this topic was Alhazen, a Basra-born scientist and mathematician who lived most of his life in Cairo, Egypt. Alhazen made innumerable contributions to the study of optics, and further refined the understanding of how mirrors and lenses could focus sunlight and generate heat.
This research continued on into the seventeenth and eighteenth centuries, as scientists experimented with various ways to refine and improve the technology. In addition to using it to ignite fires, scientists began to explore wider applications of this process. In short, how could the power of the Sun’s energy be captured, concentrated, and then applied to some other practical purpose? French engineer Salomon de Caus was one of the first to do so, using a burning glass to power a small water pump and fountain in 1615. In 1747, the French King Louis XV was presented with a mirror that could concentrate light to such a degree that its heat could melt silver. And twenty years later, Swiss scientist Horace-Bénédict de Saussure developed the first solar oven (or ‘heat box,’ as he called it), which captured the heat from the Sun’s rays through several layers of glass. The heat was sufficient to generate a temperature inside the box of 110°C (230°F). By the end of the eighteenth century, however, the practical applications of solar energy were still very limited.