AQA A Level Physics

Revision Notes

3.5.1 Refraction at a Plane Surface

Refractive Index

  • Refraction occurs when light passes a boundary between two different transparent media
  • At the boundary, the rays of light undergo a change in direction
  • The direction is taken as the angle from a hypothetical line called the normal. This is perpendicular to the surface of the boundaries and is represented by a straight dotted line
  • The change in direction depends on which media the light rays pass between:
    • From air to glass (less dense to more dense): light bends towards the normal
    • From glass to air (more dense to less dense): light bends away from the normal
    • When passing along the normal (perpendicular) the light does not bend at all

Refractive Index, downloadable AS & A Level Physics revision notes

Refraction of light through a glass block

  • The change in direction occurs due to the change in speed when travelling in different substances
    • When light passes into a denser substance the rays will slow down, hence they bend towards the normal
  • The only properties that change during refraction are speed and wavelength – the frequency of waves does not change

Calculating Refractive Index

  • The refractive index, n, is a property of a material which measures how much light slows down when passing through it

    • Where:
      • c = the speed of light in a vacuum (m s–1)
      • cs = the speed of light in a substance (m s–1)
  • Light travels at different speeds within different substances depending on their refractive index
    • A material with a high refractive index is called optically dense, such material causes light to travel slower
  • Since the speed of light in a substance will always be less than the speed of light in a vacuum, the value of the n is always greater than 1
  • In calculations, the refractive index of air can be taken to be approximately 1
    • This is because light does not slow down significantly when travelling through air (as opposed to travelling through a vacuum)

Exam Tip

Always double-check if your calculations for the refractive index are greater than 1. Otherwise, something has definitely gone wrong in your calculation!

The refractive index of air will not be given in the question. Always assume that nair = 1

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