PHYA2 - Optics

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  • Created by: Franklin
  • Created on: 13-05-14 18:18

Refraction

Refraction is the change of direction that occurs when light enters a medium non-normally  across a boundary between 2 transparent substances

The light ray bends:

  • Away from the normal if it enters a less dense medium (e.g. water to air) 
  • Towards the normal if it enters a more dense medium (e.g. glass to air)

The refractive index of air is the ratio of the speed of light in a vacuum to the speed of light in that material

refractive index equation (http://physicsnet.co.uk/wp-content/uploads/2010/08/refractive-index.jpg)

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Snell's Law

(http://www.math.ubc.ca/~cass/courses/m309-01a/chu/Fundamentals/snell01.gif)

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Total Internal Reflection (TIR)

When an incident ray is at the critical angle and enters a less dense medium, the angle of refraction = 90 degrees (refraction along the boundary)

  • TIR only occurs if the angle of incidence is greater than the critical angle
  • The material the light is going into has to have a lower refractive index for the light to bend away from the normal and towards the boundary
  • Partial internal reflection always occurs when the angle of incidence is less than or equal to the critical angle 

From more dense medium to air: 

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Why do diamonds sparkle?

  • Diamond has a high refractive index therefore the critical angle is low
  • It seperates white light more than any other substance
  • The light may be totally internally refracted many times before it leaves

(http://www.diamond-rings-info.com/images/diamond-cut01.jpg)

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Optical fibres and cladding

Optical fibres use total internal reflection to send data across a tube

  • Each time a light ray reaches the fibre boundary it is totally internally reflected
  • The incident ray of the is always greater than the critical angle of the fibre
  • Cladding increases the critical angle, protects the glass core and prevents leakage of light
  • The core should be narrow to prevent multipath dispersion

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