Refraction of Light (Cambridge O Level Physics)

• When drawing refraction ray diagrams, angles are measured between the wave direction (ray) and a line at 90 degrees to the boundary
  • The angle of the wave approaching the boundary is called the angle of incidence (i)
  • The angle of the wave leaving the boundary is called the angle of refraction (r)

• The line at right angles (90°) to the boundary is known as the normal

• When drawing a ray diagram an arrow is used to show the direction the wave is travelling
  • An incident ray has an arrow pointing towards the boundary
  • A refracted ray has an arrow pointing away from the boundary

• The angles of incidence and refraction are usually labelled i and r respectively

• The change in direction depends on which media the light rays pass between:
  • From less dense to more dense (e.g air to glass), light bends towards the normal
  • From more dense to less dense (e.g. glass to air), light bends away from the normal
  • When passing along the normal (perpendicular) the light does not bend at all

Refraction from Less Dense Medium to More Dense Medium

A ray diagram for light refracting at a boundary, showing the normal, angle of incidence and angle of refraction. The ray bends towards the normal as it is passing into a more dense medium.

Aim of the Experiment

• To investigate the refraction of light using rectangular blocks, semi-circular blocks and triangular prisms

Variables

• 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

Energy Store	Description
Ray box	to provide a narrow beam of light to refract in the perspex blocks
Protractor	to measure the angles of refraction
Sheet of paper	to mark the rays of light and the outlines of the blocks
Pencil	Magnetic interacting with each other have energy in their magnetic store
Ruler	to draw the rays of light and the outlines of the blocks
Perspex blocks (rectangular, semi-circular & prism)	to refract the rays of light

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

Ray Box and Blocks

Diagram showing a ray box alongside three different shaped glass blocks

Method

Diagram Showing Tracing Method

Apparatus to investigate refraction

1. Place the glass block on a sheet of paper, and carefully draw around the rectangular perspex block using a pencil
2. Switch on the ray box and direct a beam of light at the side face of the block
3. 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

4. Draw a dashed line normal (at right angles) to the outline of the block where the points are
5. Remove the block and join the points marked with three straight lines
6. Replace the block within its outline and repeat the above process for a ray striking the block at a different angle
7. 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

Rays Passing through the 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 
• Angles 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

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