Investigating the Refraction of Light (Oxford AQA IGCSE Physics)

Investigating the Refraction of Light

Aim of the experiment

• To investigate the refraction of light through glass and perspex blocks

Variables

• Independent variable = material of the block

• Dependent variable = angle of refraction

• Control variables:

  • Width of the light beam

  • Same frequency / wavelength of the light

Equipment

Equipment List

• Resolution of measuring equipment:

  • Protractor = 1°

  • Ruler = 1 mm

Method

Equipment for investigating the refraction of a glass or Perspex block

1. Place the Perspex or glass block on a sheet of paper, and carefully draw around it using a pencil

2. Switch on the ray box and direct a beam of light at the side face of the block

3. Mark the following 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. Remove the block and join the points marked with three straight lines

5. Draw the normal as a dashed line at a right angle to the edge of the block from the point at which the ray enters the block

6. Replace the block within its outline and repeat the above process for a ray entering the block at the normal for different angles of incidence

7. Repeat the procedure for the other material

Example Results Table

Analysis of results

• Plot a graph of sine of angle of incidence, sin(i) against sine of angle of reflection sin(r) for both the glass and Perspex blocks

• The graphs for both materials should follow the same straight line with a positive gradient

• The graph for the glass block should have a steeper gradient than the Perspex block because it has a higher refractive index

• The gradient of the graph is equal to the refractive index of the material, n because:

Angle of incidence-angle of refraction graphs for glass and Perspex

• For light rays entering the Perspex or glass block, the light ray refracts towards the normal:

i > r

• For light rays exiting the block:

  • The light ray incident on the block-to-air boundary is the same as the ray refracted from the air-to-block boundary

  • The light ray refracts away from the normal:

i < r

• So this angle of refraction is equal to the angle of incidence of the light ray in the air

• When the angle of incidence is 90° to the 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:

Measuring angles for block

Evaluating the experiment

Systematic errors:

• An error could occur if the 90° lines are drawn incorrectly

  • Use a set square to draw perpendicular lines

• Make sure the room is blacked out so it is clear to see the correct ray lines

Random errors:

• The points for the incident ray and reflected ray 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