Core Practical 4: Investigating Refraction
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
- Place the glass block on a sheet of paper, and carefully draw around the rectangular perspex block using a pencil
- Switch on the ray box and direct a beam of light at the side face of the block
- 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
- Draw a dashed line normal (at right angles) to the outline of the block where the points are
- Remove the block and join the points marked with three straight lines
- Replace the block within its outline and repeat the above process for a ray striking the block at a different angle
- 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