Core Practical: Investigating Refraction (Edexcel IGCSE Physics: Double Science)

Aim of the experiment

• To investigate the refraction of light using transparent rectangular blocks, semi-circular blocks and triangular prisms
  • To review your understanding of refraction use the revision note Reflection & refraction

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

Equipment	Purpose
Ray Box	To provide a narrow beam of light that can be easily refracted
Protractor	To measure the angles of incidence and refraction
Sheet of Paper	To mark the lines indicating the incident and refracted rays
Pencil	To draw the incident and refracted ray lines onto the paper
Ruler	To draw the incident and refracted ray lines onto the paper
Perspex blocks (rectangular block, semi-circular block & prism)	To refract the light beam

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

Method

Refraction experiment set up

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)

Results

• Consider the light paths through the different-shaped blocks

Refraction experiment results with different media

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 
• Use the laws of refraction to analyse these results
  • You can use the revision note Reflection & refraction to do this

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 minutes

• 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