Answer
The choice between using geometric optics or the wave model depends on the size of the object being illuminated, as compared to the wavelength of the light.
Work Step by Step
Geometric optics is useful for studying lenses and mirrors, which are much larger than the wavelength of light.
The more complicated wave nature of light must be taken into account when dealing with smaller objects (e.g., slits, thin films) that are about the size of the wavelength of light.
