Reflection & Refraction (Edexcel IGCSE Physics)

• 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

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

Waves can change direction when moving between materials with different densities

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

Angle of incidence and angle of reflection

Reflection Ray Diagrams

• Angles are measured between the wave direction (ray) and a line at 90 degrees to the boundary
  • 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 law of reflection states that these angles are the same

Ray diagram of reflection of a wave at a mirror

• 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

• The angles of incidence and reflection are usually labelled i and r respectively and measured from the normal

Refraction Ray Diagrams

• Refraction 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 a hypothetical line called the normal
  • This line is perpendicular to the surface of the boundaries and is usually represented by a straight dashed or dotted line

• 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

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 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

Step 1: Draw a parallel ray on the left

Step 2: Draw the refracted ray at the first surface

• • As the ray enters the block it bends towards the normal since it is going into a denser material
  • In this case, the angle of refraction is smaller than the angle of incidence

Step 3: Draw the refracted ray at the second surface

• • As the ray leaves the block it bends away from the normal
  • In this case, the angle of refraction is larger than the angle of incidence