Refraction at a plane surface GapFill

When a wave meets a boundary between two media at an angle, the direction of propagation of the wave changes. This is known as refraction, and occurs because of a change in the  frequencyphasespeedpolarisation of the wave.
The degree of refraction depends on the  refractive indexrefractivitycritical anglerefractive dispersion, n, of both media, and for air this is  infinityexactly 12approximately 1.
The change in angle due to refraction can be determined from Snell's law:  n₁ ÷ sin(θ₁) = n₂ ÷ sin(θ₂)n₁ × n₂ = sin(θ₁) × sin(θ₂)n₁ × sin(θ₁) = n₂ × sin(θ₂)θ₁ × sin(n₁) = θ₂ × sin(n₂).
When a wave reaches a boundary above the  Snell's anglerefractive indexcritical angletotal internal angle, total internal reflection occurs, and the wave is reflected back into the first medium. This can only occur when n₁ is  lower thangreater thanthe inverse ofmuch lower than n₂.
Optical fibres take advantage of total internal reflection to transmit light. Optical fibres consist of a core and cladding, which have carefully selected values of n to ensure total internal reflection occurs in the majority of cases.
There are a number of ways in which signals along optical fibres can be lost or in some way corrupted.
 Modal dispersionDiffractionInterferenceMaterial dispersion occurs due to different wavelengths travelling at different speeds through the core, so the pulses travel at different speeds and spread out.
 DiffractionMaterial dispersionInterferenceModal dispersion occurs due to the pulses travelling down the core and reflecting off the core-cladding boundary at different angles, so they travel different path distances and spread out.
 AbsorptionInterferencePulse broadeningDiffraction occurs due to impurities in the optical fibre, causing a loss of intensity of the signal.