3.5.1 Refraction at a Plane Surface

• 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 line 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

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
  • Technically the intensity and amplitude of the wave also decreases slightly upon refraction since a small portion of the wave also reflects but this is not as noticeable as the change in speed and wavelength

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)
    • c_s = 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)