Optical model for the description of the behaviour of lenses and other optical parts.
We can only use this model , if the wavelengths involved are very small compared to the equipment used for their study. Further the photon energy (see quantum theory) must be small compared with the energy sensitivity of the equipment.
Geometrical optics ignores diffraction and interference.
Geometrical optics can not be used for radio waves, because the wavelengths are larger than the equipment.
If wavelengths are in the region of the dimensions of the equipment (hard to reach with visible light, but easy with radio waves), and photo energies are negligibly small, then an approximation by study of the waves and disregarding quantum effects is appropriate called “classical theory of electromagnetic radiation”.
If wavelengths get very small, we can ignore the wave character of the light and photons have a large energy compared to the sensitivity of the equipment things get simple again, we get the simple photon picture.
The complete picture unifies everything into one model and is too complicated here.
compare to first order optics.