Final answer:
The effective refractive index can be less than the cladding's in some specialized waveguide structures, although in standard optical fibers, this is not the case to ensure total internal reflection.
Step-by-step explanation:
When discussing waveguides and their effective refractive index (NA), in some cases the effective refractive index can be less than that of the cladding if the waveguide width is very small, such as in some plasmonic waveguides. This is because the mode of the light may extend significantly into the cladding or surrounding medium.
However, for standard optical fibers, the waveguide — the core of the fiber — typically has a higher refractive index compared to the cladding to ensure total internal reflection. Therefore, the waveguide's NA cannot be less than that of the cladding since this would allow light to escape, defeating the purpose of the waveguide.
For situations employing immersion lenses, a medium like oil or water with a refractive index closer to the objective's can increase the NA above 1.00, allowing more light into the lens and increasing resolution.