Total Internal Reflection

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Total Internal Reflection

  • The ray is reflected along the water and air boundary. At this point θ1 is known as the critical angle θc
  • The ray is reflected back at the water boundary. This partial reflection where θ3 = θ1

(http://www.schoolphysics.co.uk/age11-14/Light/text/Total_internal_reflection/images/3.png)

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Total Internal Reflection 2

  • The law of refraction states that n1sinθ1 =n2sinθ2
  • When the refracted ray passes directly across the boundary the incident ray is striking the boundary at the critical angle θc
  • n1sinθ1 = n2sinθ2
  • Since sin90 = 1, n1sinθc = n2
  • Sinθc = n2/n1
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Refraction in Optical Fibres

  • The light is refracted as it enters the fibre. Since the refractive index of air is 1: Sinθ = (1/n2) sinθ where n2 is the refractive index of the glass (usually about 1.5)
  • In effect, θ1 > θ2 so the ray refracts towards the normal
  • The ray proceeds inside the fibre until it is incident upon the boundary. Provided that the angle of incidence at this event (θ3 = 90 - θ2) is greater than the critical angle, total internal reflection will occur and the ray will be trapped inside the fibre,
  • (http://core.physicsinfo.co.uk/filestore/1314/IMG_7219_-_Copy.JPG)
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Structure of Optical Fibres

  • The simplest kind of optical fibre is a thin piece of glass or other transparent medium
  • An effective improvement on the simple fibre is to wrap the core fibre in a material known as cladding. The cladding protects the core from scratches or moisture that may enable light to leak out. This allows information to be transmitted with a greater degree of security.
  • Multimodal dispersion occurs in simple optical fibres with a low critical angle (about 42) as there are many paths that the ray can follow. For example in an extreme case, yellow reaches the reciever before the red ray (which has travelled further)
  • To reduce multimodal dispersion you can use monochromatic light and increase the critical angle by making the core thinner and the cladding thicker
  • Coherent Signals- What is coming in is the same as what is coming out.
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