Light guidance in an optical fiber can be understood by considering a structure comprising of thin solid glass cylinder of refractive index $$n_1$$ surrounded by a medium of lower refractive index $$n_2$$. The light guidance in the structure takes place due to successive total internal reflections at the
interface of the media $$n_1$$ and $$n_2$$ as shown in the figure. All rays with the angle of incidence i less than a particular value $$i_m$$ are confined in the medium of refractive index $$n_1$$. The numerical aperture (NA) of the structureis defined as sin $$i_m$$.
For two structures namely $$S_1$$ with $$n_1 = \frac{\sqrt 45}{4}$$ and $$n_2 = \frac{3}{2}$$, and $$S_2$$ with $$n_1 = \frac{8}{5}$$ and $$n_2 = \frac{7}{5}$$ and taking the refractive index of water to be $$\frac{4}{3}$$ and that of air to be 1, the correct option(s) is(are)
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