Optics deals with the properties of radiation, in particular with the propagation of light and the interaction with matter.

In the field of geometrical optics, radiation is described by linear beams. With this idealization, the principle of telescopes or microscopes can be explained, for example, with the aid of the law of refraction and the law of reflection.
If, on the other hand, one considers optical systems in which the dimensions lie in the range of the wavelength of the radiation, then phenomena occur which can no longer be explained with geometric optics, e.g. diffraction at slits or gratings.

This is where physical optics (wave optics) comes in, which explicitly treats radiation as a wave phenomenon.