What is the difference between physical optics and geometric optics?
Geometric optics is when we treat light as a single beam (A ray) and study the properties. It deals with lenses, mirrors, phenomenon of total internal reflection, formation of rainbows, etc etc. In this case, the wavelike properties of light become insignificant as the objects we deal with are very huge as compared to the wavelength of light.
But, in physical optics, we consider the wave like properties of light and develop the more advanced concepts on the basis of Huygen's principle. We would deal with Young's double slit experiment and consequently with interference of light which is a characteristic of waves. We also deal with polarization and Diffraction which are also typical wavelike properties. Diffraction happens only when the obstacle's size is of the order of the wavelength of light. Maxwell's electromagnetic theory put the wave theory of light on a very firm footing. It is to be noted that reflection and refraction are explained by physical optics as well.
That's the basic difference was this.
In the later half of the 19th century, properties of radiation was discovered which could only be explained considering that radiation was comprised of discrete packets of energy. (Light is also a radiation).
So, whether a wave or a particle description is best depends on the situation.