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

Core practical 5: investigating snell's law

Aims of the experiment

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

Variables

• 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

Equipment list

• Resolution of measuring equipment:

  • Protractor = 1°

  • Ruler = 1 mm

Method

Diagram of equipment set up

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 results table

Analysis of results

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

A diagram showing how to measure the angles of incidence and refraction

• Snell's Law relates the angles of incidence and refraction

  • This is covered in the Snell's law revision note

• 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

A graph of the results of snell's law experiment

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