At passing of light ray on transparent medium with the different values of transmission density (n1 > n2) there are phenomena of reflection and refraction of light. The refractive index n of medium is name the relation of velocity of light in a vacuum to velocity of light in this medium. Refractive index for every homogeneous material or substance is constant. For silica there is an refractive index about 1,5.

In passage of ray from an medium with a greater transmission density in a medium with a less transmission density (n₁ > n₂) the corner of refraction will exceed the light incidence angle (γ > α ). Due to the increase of light incidence angle α it is possible to reach the effect of total internal reflection, when light will not pass the boundary of mediums (γ≥90°), based on basis principle of light transmission through optical fibre. Light incidence angle, on which an effect of total internal reflection there is, named the critical angle of light input in optical fibre.

Consequently, in order that a ray propagate only in the limited medium it is necessary : to enter a ray with angle no more than critical angle, to use for the light transmission two mediums with a different transmission densities (refractive index ).

For this reason optical fibre consists of core with transmission density more than claddinng transmission density.

The maximum permissible angle of light input in fibre named acceptance angle, the sine of this angla often reported as a numerical aperture, which is calculated on a formula:, where θ_max - is the acceptance angle, n₁ - is an core refractive index, n₂ - is an refractive index of the fibre cladding.