Reflection & Refraction (Edexcel IGCSE Physics)

Spec point — 3.9 · explain that all waves can be reflected and refracted

Reflection & refraction

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

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

• In optics the word medium is used to describe a material that transmits light

  • Media means more than one medium

An example of reflection

An identical image of the tree is seen in the water due to reflection

• Refraction occurs when: A wave passes a boundary between two different transparent media and undergoes a change in direction

An example of refraction

Waves can change direction when moving between materials with different optical densities

The law of reflection

Spec point — 3.15 · use the law of reflection (the angle of incidence equals the angle of reflection)

• The law of reflection states that: Angle of incidence (i) = Angle of reflection (r)

• Angles are measured between the wave direction (ray) and a line at 90 degrees to the boundary called the normal

  • 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)

An example of reflection in a plane mirror

Ray diagram of the reflection of a wave in a mirror

Ray diagrams

Spec point — 3.16 · draw ray diagrams to illustrate reflection and refraction

Reflection ray diagrams

• 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 reflected ray has an arrow pointing away from the boundary

A diagram showing the law of reflection

The angle of incidence and angle of reflection are equal in the law of reflection

Refraction ray diagrams

• The direction of the incident and refracted rays are also taken from the normal line

• The change in direction of the refracted ray depends on the difference in optical density between the two media:

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

A diagram of a ray refracted into and out of 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 optically 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

Exam Hint When drawing ray diagrams for reflection:

1. A simple straight line with an arrow is enough to represent the wave

• You do not need to draw the wavefronts unless asked to do so!

1. Take care to draw the angle correctly

• If it is slightly out it won’t be a problem, but if there is an obvious difference between the angle of incidence and the angle of reflection then you will probably lose a mark!

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

Don't forget to draw the arrows for the direction of the light rays and make sure they are drawn with a ruler and a sharp pointed pencil