Reflection & Refraction of Waves (WJEC GCSE Physics)

• All waves, whether transverse or longitudinal, can be reflected and refracted

Reflection

• Reflection occurs when:

A wave hits a boundary between two media and does not pass through, but instead stays in the original medium

• Angles are measured between the wave direction (ray) and the normal
  • The normal is perpendicular to the surface of the boundaries and is usually represented by a straight line or dotted line
  • The angle of the wave approaching the boundary is called the angle of incidence (i)
  • The angle of the wave leaving the boundary is called the angle of reflection (r)
• The angles are the same, so the law of reflection can be written:

Angle of incidence (i) = Angle of reflection (r)

Light Reflecting From a Surface

The angle of incidence and angle of reflection are the same when light reflects from a surface

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 of water
  • In some cases, the wave will change direction

Refraction of water waves

• Refraction can occur when water waves travel from deep water to shallow water
• This effect can be observed using water waves in a ripple tank
• Refraction can be represented using wavefront diagrams, as shown below:

Water Waves Refracting in a Ripple Tank

The different parts of the wave enter a different depth of water at different times causing the wave to bend

• When water waves pass from deep water to shallow water:
  • The wave speed decreases
  • The wavelength decreases
  • The frequency remains the same

• The difference in speed between the parts of the wave in deep water and the parts in the shallow water causes the wave to bend
  • Hence, this leads to a change in direction
• If the boundary between the deep water and shallow water is perpendicular to the wavefront, the whole wavefront will change speed at the same time and the wave will not change direction

Refraction of light

• Refraction also 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 the normal
• The 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 Prism

Light enters the glass prism where the light ray bends towards the normal. Light bends away from the normal as it exits the glass prism

• 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

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