refractive index

• the refractive index of a material measures how much it slows down light.
• light travels fastest in a vacuum, and slows down in other materials as it interacts with the particles in them. the more optically dense a material is, the more light slows down when it enters it.
• the absolute refractive index of a material, symbol = n, is a measure of optical density. it is found from the ratio between the speed of light in a vacuum, and the speed of light in that material.
• the relative refractive index between two materials, is the ratio of the speed of light in one material to the speed of light in another material
• the absolute refractive index of a material is a property of that material only.

  a relative refractive index is a property of the boundary between two materials.
• the angle the incoming light makes to the normal is the angle of incidence.

  the angle the refracted ray makes with the normal is the angle of refraction.
• snells law states that

  $n1\sin\theta1\ =$$\ n2\sin\theta2$

  • $n1$ = refractive index of first material
  • $\theta1$ = angle of incidence
  • $n2$ = refractive index of second material
  • $\theta2$ = angle of refraction
• the critical angle is the angle of incidence when the angle of refraction is 90 degrees
• to find the critical angle, shine a ray of light at a boundary from a more optically dense material into a less optically dense material. the light will be refracted away from the normal. if you gradually increase the angle of incidence, the angle of refraction will gradually get closer and closer to 90 degrees.
• total internal reflection is when all the light is reflected back into the first material. this happens when the angle of incidence is greater than the critical angle
• an optical fibre is a thin flexible tube of glass or plastic fibre that can carry light signals over long distances and round corners.
• step index optical fibres have a high refractive index but are surrounded by cladding with a lower refractive index to allow for total internal reflection. cladding also protects the fibre from scratches which could let light escape
• in step index optical fibres, light is shone in at one end of the fibre. the fibre is so narrow that the light always hits the boundary between the fibre and cladding at an angle bigger than the critical angle, so all the light is totally internally reflected from boundary to boundary until it reaches the other end.

  
• a signal (a stream of pulses of light) travelling down an optical fibre can be degraded by absorption or by dispersion. signal degradation can cause information to be lost
• as the signal travels down the optical fibre, some of its energy is lost through absorption by the material the fibre is made from. this energy loss results in the amplitude of the signal being reduced
• there are two types of dispersion that can degrade a signal:

  • modal dispersion
  • material dispersion
• modal dispersion is when light rays enter the fibre at different angles, so take different paths. the rays which take a longer path take longer to reach the other end than those that travel down the middle of the fibre. a single mode fibre only lets light take one path, so it stops modal dispersion
• material dispersion is when light consists of different wavelengths that travel at different speeds in the fibre - this causes some light wavelengths to reach the end of the fibre faster than others. using monochromatic light can stop material dispersion.
• both types of dispersion (modal and material) lead to pulse broadening. the signal sent down the fibre is broader at the other end. broadened pulses can overlap each other and confuse the signal.
• an optical fibre repeater can be used to boost and regenerate the signal every so often, which can reduce signal degradation caused by both absorption and dispersion