Core Practical: Investigating Refraction (Edexcel IGCSE Physics)

Aim of the Experiment

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

• 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

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 
• 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

• If the experiment was carried out correctly, the angles should follow the pattern, as shown below:

How to measure the angle of incidence and angle of refraction

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