JEE Advanced 2015 Paper 2

Paragraph for next 2 Questions Light guidance in an optical fiber can be understood by considering a structure comprising of thin solid glasscylinder of refractive index $n_1$ surrounded by a medium of lower refractive index $n_2$. The light guidance in thestructure takes place due to successive total internal reflections at the interface of the media $n_1$ and $n_2$ as shown inthe figure. All rays with the angle of incidence i less than a particular value $i_{m}$ are confined in the medium ofrefractive index $n_1$. The numerical aperture (NA) of the structure is defined as sin $i_{m}$.