Unit 2 Section 3 Critical Angle and TIR


Critical Angle

Critical Angle: The angle of incidence going in, at which the angle of refraction going out is 90 degrees to the normal, refracted along the boundary.

1. Critical Angle

sin(theta,c) = n2 / n1

  • theta,c = critical angle
  • n2 = refractive index of less optically dense material
  • n1 = refractive index of more optically dense material

2. Refractive Index of Material

n1 = 1 / sin(theta,c)

  • n1 = refractive index of the material
  • theta,c = critical angle for material to air boundary
  • n2 = 1
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Total Internal Reflection

TIR: When all light is completely reflected back into a medium at a boundary with another medium, instead of being refracted. It only happens at angles of incidence greater than the critical angle.


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Optical Fibres

Optical Fibre: A thin flexible tube of glass or plastic that can carry light signals using total internal reflection.

  • Core of the fibre has a high refractive index and is optically dense.
  • Core is surrounded by cladding with a lower refractive index and is less optically dense.
  • Light through optic fibres is used to transmit phone and cable tv signals.

Advantages over the old electric copper cable system:

  • The signal can carry more information because light has a high frequency.
  • The light doesn't heat up the fibre - so almost no energy is lost as heat.
  • There is no electrical interference.
  • They are much cheaper to produce .
  • The signal can travel a long way, very quickly and with minimal amounts of signal loss.
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Signal Degradation in Optical Fibres

Some energy is lost through absorption and scattering by the material the optical fibre is made from.

Energy is also lost if any light escapes the fibre.

The energy loss results in a reduced signal amplitude.

A signal booster can be used to reverse this effect.

A signal that travels straight down the middle of a fibre (axial ray) will take less time than one which bounces off of the sides (non axial ray).

This means parts of the signal will arrive at the end of the fibre before the rest, and so the signal is broadened which can cause a loss in quality if several signals broaden and merge together.

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