Euclid (3rd century BC) was an ancient Greek scholar. The biographical information about him is very limited; it is known only that he worked in Alexandria at the beginning of the 3rd century BC. He is the author of the first treatise on mathematics that has come down to us (“Elements”), which summed up the preceding development of ancient Greek mathematics and set out, in particular, plane geometry, solid geometry and a number of questions in the theory of numbers. He was the creator of the geometrical system (Euclidean geometry) on which the whole of classical physics rests.
Euclid's research in optics is set out in his treatises “Optics” and “Catoptrics”. Following Plato, he accepted the theory of visual rays (these rays are straight lines). He formulated the law of the rectilinear propagation of light and the law of the reflection of light. In his works he examined the formation of shadow, the production of images by means of small apertures, and the phenomena connected with reflection from plane and spherical mirrors. All this gives grounds for regarding Euclid as the founder of geometrical optics.