Investigating the Refraction of Light (Oxford AQA IGCSE Physics)

Revision Note

Ann Howell

Written by: Ann Howell

Reviewed by: Caroline Carroll

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

Equipment

Purpose

Ray box

To provide a narrow beam of light to reflect in the mirror

Protractor

To measure the angle of the light beams

Sheet of paper

To mark the light rays

Pencil

To mark the paper

Ruler

To draw lines on the paper

Perspex block

To refract the light

Glass block

To refract the light

  • Resolution of measuring equipment:

    • Protractor = 1°

    • Ruler = 1 mm

Method

Equipment for investigating the refraction of a glass or Perspex block

The light ray box provides the light ray that enters the perspex or glass block at different angles of incidence.
Apparatus to investigate refraction
  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

Material of block

angle of incidence, (i) / °

angle of refraction, (r) / °

sin(i)

sin(r)

Perspex

10

Perspex

20

Perspex

30

Perspex

40

Perspex

50

Perspex

60

Perspex

70

Glass

10

Glass

20

Glass

30

Glass

40

Glass

50

Glass

60

Glass

70

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:

n space equals space fraction numerator sin i over denominator sin r end fraction

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

Sin i is plotted on the vertical axis and sin r on the horizontal axis, for IGCSE & GCSE Physics revision notes
The gradient of the angle of the sini/sinr graph represents the refractive index of the material, n
  • 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

The angle of incidence and the angle of refraction are both measured against the normal line at right angles to the block.
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

  • 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

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Ann Howell

Author: Ann Howell

Expertise: Physics Content Creator

Ann obtained her Maths and Physics degree from the University of Bath before completing her PGCE in Science and Maths teaching. She spent ten years teaching Maths and Physics to wonderful students from all around the world whilst living in China, Ethiopia and Nepal. Now based in beautiful Devon she is thrilled to be creating awesome Physics resources to make Physics more accessible and understandable for all students, no matter their schooling or background.

Caroline Carroll

Author: Caroline Carroll

Expertise: Physics Subject Lead

Caroline graduated from the University of Nottingham with a degree in Chemistry and Molecular Physics. She spent several years working as an Industrial Chemist in the automotive industry before retraining to teach. Caroline has over 12 years of experience teaching GCSE and A-level chemistry and physics. She is passionate about creating high-quality resources to help students achieve their full potential.