Refraction (Oxford AQA IGCSE Physics)

Explaining Refraction

• Refraction is:

The change in direction of a wave which is 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

  • If the incident wave hits the boundary perpendicularly (at 90°) the wave will not change direction

• 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

• So it follows that 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

Refraction of Light

• Refraction also occurs when light passes a boundary between two different transparent media

• At the boundary, the rays of light change direction

  • The direction is taken as the angle between the ray and the normal

• This change in direction depends on the difference in density between the two media:

  • From less dense to more dense (e.g air to glass), light bends towards the normal

  • From more dense to less dense (e.g. glass to air), 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

• As with the refraction of water waves, the only properties that change during the refraction of light 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

• At any boundary:

  • The incident ray is the ray approaching the boundary

  • The refracted ray is the ray leaving the boundary

• There are two boundaries as light passes through a glass block

  • Air to glass

  • Glass to air

• At the air-to-glass boundary:

  • The incident ray is the ray in the air

  • The refracted ray is the ray in the glass

• At the glass-to-air boundary:

  • The incident ray is the ray in the glass

  • The refracted ray is the ray in the air