Explain Total internal reflection?

The refractive index of a medium is defined as the ratio of velocity of light in vacuum to the velocity of light in that medium

n_x = C/V

where nx is the refractive index of the medium, C is the velocity of light in vacuum and V is the velocity of light in in which medium.
Consider two mediums of refractive indices n1 and n2, where n1>n2. When a ray travels from n1 (denser) to n2 (rarer), the refracted ray bends more away from the normal. This is display below:

According to Snell's law of refraction
n1 sin O1 = n2 sin O2
sin O1/ sin O2= n2/n1
when we make the angle of incidence as some Oc such that O2=900. This angle Oc is known as Critical angle. The condition is shown below:

When the incident angle increases from Oc, the light is reflected back within the originating dielectric medium. This phenomenon is known as total internal reflection. Therefore it may be observed that total internal reflection occurs at the interface among two dielectrics of differing refractive indices, while light is incident on the dielectric of lower index from the dielectric of higher index. The total internal reflection is shown below:

The light ray passing by the optic fibre follows the total internal reflection at the core-cladding interface.