When was the law of reflection discovered?

Answer 1

In antiquity. A statement of it has been attributed to Euclid, although his authorship of the book in question is disputed. Be that as it may, the law was certainly known to Heron of Alexandria in the first century. Heron even showed that reflection according to this law yielded the shortest reflected path between two fixed points. The first explanation of the law in terms of a wave theory of light was given by Huygens in the 17th century.