Core Practical: Investigating Snell's law (Edexcel IGCSE Physics)

Aim of the Experiment

• To investigate the refractive index of glass, using a glass block

• Independent variable = angle of incidence, i
• Dependent variable = angle of refraction , r
• Control variables:
  • Use of the same perspex block
  • Width of the light beam
  • Same frequency / wavelength of the light

Equipment List

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

Method

Apparatus set-up to investigate Snell's Law

1. Place the glass block on a sheet of paper, and carefully draw around the block using a pencil
2. Draw a dashed line normal (at right angles) to the outline of the block
3. Use a protractor to measure the angles of incidence to be studied and mark these lines on the paper
4. Switch on the ray box and direct a beam of light at the side face of the block at the first angle to be investigated
5. 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

6. Remove the block and join the points marked with three straight lines
7. Replace the block within its outline and repeat the above process for a rays striking the block at the next angle

• An example table of results might look like this:

Analysis of Results

• If the angles have been measured correctly, the paper should end up looking like this:

• The angles of incidence and refraction are related by Snell's Law:

• Where:
  • n = refractive index of the material
  • i = angle of incidence of the light (°)
  • r = angle of refraction of the light (°)

• Plot a graph of sin i on the y-axis against sin r on the x-axis
  • The refractive index is equal to the gradient of the graph

• An example graph might look like 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 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