Refraction of Light (Cambridge (CIE) IGCSE Co-ordinated Sciences (Double Award)): Revision Note

Defining refraction

• Refraction is defined as:

  The change in direction of a light ray passing from one medium to another

• In refraction, angles are measured between the ray (showing the direction of the wave) and the normal 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 refraction (r)

• When drawing a ray diagram an arrow is used to show the direction the wave is travelling

  • An incident ray has an arrow pointing towards the boundary

  • A refracted ray has an arrow pointing away from the boundary

• The angles of incidence and refraction are usually labelled i and r respectively

Refraction ray diagram

A ray diagram for light refracting at a boundary, showing the normal, angle of incidence and angle of refraction

Refraction of light

• 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

• Note that when a light wave enters and leaves the glass block there are two boundaries

  • The refracted ray at the first boundary becomes the incident ray at the second boundary

Refraction diagram of light from air through a glass block

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 waves 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