Refraction of Light (CIE IGCSE Physics)

• When drawing refraction 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 refraction (r)

• The line at right angles (90°) to the boundary is known as the normal

• 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

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

• 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 which media the light rays pass between:
  • 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

Aim of the Experiment

• To investigate the refraction of light using rectangular blocks, semi-circular blocks and triangular prisms

Variables

• Independent variable = shape of the block
• Dependent variable = direction of refraction
• Control variables:
  • Width of the light beam
  • Same frequency / wavelength of the light

Equipment List

• Resolution of measuring equipment:
  • Protractor = 1°
  • Ruler = 1 mm

Diagram showing a ray box alongside three different shaped glass blocks

Method

Apparatus to investigate refraction

1. Place the glass block on a sheet of paper, and carefully draw around the rectangular perspex block using a pencil
2. Switch on the ray box and direct a beam of light at the side face of the block
3. Mark on the paper:
   • A point on the ray close to the ray box
   • The point where the ray enters the block
   • The point where the ray exits the block
   • A point on the exit light ray which is a distance of about 5 cm away from the block

4. Draw a dashed line normal (at right angles) to the outline of the block where the points are
5. Remove the block and join the points marked with three straight lines
6. Replace the block within its outline and repeat the above process for a ray striking the block at a different angle
7. Repeat the procedure for each shape of perspex block (prism and semi-circular)

Analysis of Results

• Consider the light paths through the different-shaped blocks

Refraction of light through different shapes of perspex blocks

• The final diagram for each shape will include multiple light ray paths for the different angles of incidences (i) at which the light strikes the blocks
• This will help demonstrate how the angle of refraction (r) changes with the angle of incidence 
  • Label these paths clearly with (1) (2) (3) or A, B, C to make these clearer 
• Angles i and r are always measured from the normal
• For light rays entering the perspex block, the light ray refracts towards the central line:

i > r

• For light rays exiting the perspex block, the light ray refracts away from the central line:

i < r

• When the angle of incidence is 90° to the perspex block, the light ray does not refract, it passes straight through the block:

i = r

Evaluating the Experiment

Systematic Errors:

• An error could occur if the 90° lines are drawn incorrectly
  • Use a set square to draw perpendicular lines

Random Errors:

• The points for the incoming and reflected beam may be inaccurately marked
  • Use a sharpened pencil and mark in the middle of the beam

• The protractor resolution may make it difficult to read the angles accurately
  • Use a protractor with a higher resolution

Safety Considerations

• The ray box light could cause burns if touched
  • Run burns under cold running water for at least five minute

• Looking directly into the light may damage the eyes
  • Avoid looking directly at the light
  • Stand behind the ray box during the experiment

• Keep all liquids away from the electrical equipment and paper