Comment by CliffStoll

Indeed! We like to think of light as photons, traveling in rays in straight lines. That's geometric optics and works great for most purposes in photography.

Physical optics takes into account the wave nature of light. This becomes important when the size of the lens becomes small (eg pinholes) ... there's diffraction around edges and pixels receive contributions from many points in space.

Geometric optics lets you model using ray-tracing, reflections, and Snell's law refraction.

Physical optics uses tools such as Fourier transforms, convolutions, and sinc functions.

Understanding a simple lens system? Geometric optics is your friend. Building an astronomical telescope? Check into physical optics.