Is a ray diagram a type of diagram?

Answer 1

If diffraction effects are not relevant, that is if no dimension of the system under analysis is of the order of magnitude of a few the light wavelength (that is few micron), light propagation can be approximated as the propagation of light rays. The propagation of light rays is regulated by the rays equation that is similar to the Hamilton-Jacobi form of the classical mechanics motion equations.

The ray diagram in this case is the set of trajectories of the light rays.

It is to underline that the condition that no dimension of the considered system is of the order of a few microns is not so easy to satisfy. Also in macroscopic systems, very small dimensions are present near abrupt angles or very small holes so that diffraction effects, that cannot be modelled by the approximation of light rays, are important sometimes also in macroscopic systems. Examples are the maximum resolution of a photographic machine (the resolution that is attained if an infinite number of digital pixels are given, that is a condition very well verified by several real machines) that is ultimately determined by the light diffraction through the objective opening or the fine structure of diffraction light we can generally observe at the edge of windows glasses especially when the sun is low in the sky.