Refraction (HT Only) (AQA GCSE Combined Science: Synergy: Life & Environmental Sciences): Revision Note

Refraction

• Refraction is defined as:

The change in direction of a wave caused by a change in the speed of the wave

• Refraction can occur when a wave crosses a boundary between two materials with different densities or when water passes over a boundary of different depths

• If the incident wave hits the boundary:

  • At an angle, the wave will change direction

  • Perpendicularly (at 90°) the wave will not change direction

Refraction ray diagrams

• 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 usually represented by a straight dashed or dotted line

• The change in direction depends on which media the light rays pass between:

• When passing along the normal (perpendicular) the light does not bend at all

How to construct a ray diagram showing the refraction of light as it passes through a rectangular 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

  • Different frequencies account for different colours of light (red has a low frequency, whilst blue has a high frequency)

  • When light refracts, it does not change colour (think of a pencil in a glass of water), therefore, the frequency does not change

Wavefront Diagrams

• When an incident wave hits a boundary at an angle, one end of the wavefront changes speed before the other end of the wavefront

  • This causes the wavefront to change direction

  • This can be seen most easily on a wavefront diagram

Refraction of a wave at a boundary

• When waves pass from a less dense medium to a denser medium:

  • The wave speed decreases

  • The wavelength decreases

  • The frequency remains the same

• When a wave hits a boundary perpendicularly, the entire wavefront changes speed at the same time

  • In this case, the wave does not change direction

Refraction of water waves

• Refraction can occur when water waves travel between deep water and shallow water

• This effect can also be observed using water waves in a ripple tank

Water waves refracting in a ripple tank

• When water waves pass from deep water to shallow water:

  • The wave speed decreases

  • The wavelength decreases

  • The frequency remains the same