The optical fibers have in an inner core of refractive index $n_1$ and a cladding of refractive index $n_2$, such that:

$n_1 > n_2$

In an optical fibre, the inner core is more dense than the cladding. The function of the cladding is to provide a lower refractive index at the core interface in order to cause reflection within the core so that light waves are transmitted through the fiber.

so $n_1 > n_2$