Main Question or Discussion Point
How is the "geometric optics approximation" exactly defined? Given all the source of visible radiation's parameters, all the apparatus, instruments, screen, etc, specifications, how can I know if, e. g. there will be diffraction, interference or other wave properties or if I'll be able to describe the radiation's behaviour only with the geometric optics law? Usually, in the textbooks, I find this definition: "...when the wavelength is small" but compared to what? Or I find "... when the wavelength is small compared to the (smallest of the) object's dimensions", which is not even correct.