Optics is the science which treats of the propagation of light, and is usually divided into geometrical and physical optics. Geometrical optics treats of those simple laws of the propagation of light which have been experimentally established, and uses these laws in the solution of more complex problems; physical oj)tics explains experimental facts by showing that they agree with certain hypotheses on the structure of matter and space. (For physical optics see Light.) In elementary geometrical optics it is usual to consider light as emanating from a mathematical point and as being made up of rays extending in all directions. These rays always travel in straight lines through any one uniform medium, unless rays from different origins interfere with each other, as in the phenomenon of diffraction (q.v.). When light falls on any object some of it is irregularly reflected or scattered in all directions, since the surface is not absolutely smooth, but composed of an infinite number of tiny excrescences; this scattering renders the object luminous and visible. From a smooth surface the rays are thrown back in one direction only and obey the laws of reflection (q.v.). If the object is of such a nature that light can enter it at all, the rays are bent in accordance with the laws of refraction (q.v.). Many problems of interest arise from a consideration of the paths of rays after reflection or refraction at plane or spherical surfaces. Of the greatest practical importance, however, are the problems relating to the behaviour of rays after refraction through translucent bodies known as prisms and lenses (q.v.), since upon this behaviour depends the utility of the various optical instruments, such as the microscope, telescope, etc.