Refraction of Light (CIE IGCSE Physics: Co-ordinated Sciences (Double Award))

Revision Note

Ann H

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Ann H

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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

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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

Refraction of Light, downloadable IGCSE & GCSE Physics revision notes

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

Examiner Tip

Practice drawing refraction diagrams as much as you can! It's very important to remember which way the light bends when it crosses a boundary:

As the light enters the block it bends towards the normal line

Remember: Enters Towards

When it leaves the block it bends away from the normal line

Remember: Leaves Away

You only need to know about light passing through the boundaries between two media.

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Ann H

Author: Ann H

Expertise: Physics

Ann obtained her Maths and Physics degree from the University of Bath before completing her PGCE in Science and Maths teaching. She spent ten years teaching Maths and Physics to wonderful students from all around the world whilst living in China, Ethiopia and Nepal. Now based in beautiful Devon she is thrilled to be creating awesome Physics resources to make Physics more accessible and understandable for all students no matter their schooling or background.