Question

refractive index of cladding is _ then refractive index of core in a optical fibre

Answer 1

The refractive index (n) of a material is the ratio of the speed of light (c) in a vacuum to the velocity of light in the material (cS).

n = c/cs 

The refractive index of a material is always greater than 1.

For example;

water = 1.33

diamond = 2.42

glass = 1.5

air » 1

When a ray of light goes from material (1) into material (2), rather than from a vacuum into a material we talk about the relative refractive index.