Refraction of Light | Physics for GCSE/IGCSE - Year 11 PDF Download

Ray Diagrams for Refraction

• When illustrating refraction ray diagrams, angles are measured between the direction of the wave (ray) and a line perpendicular to the boundary.
  • The angle of incidence (i) is the angle at which the wave approaches the boundary.
  • The angle of refraction (r) is the angle at which the wave leaves the boundary.
• The line perpendicular (at 90 degrees) to the boundary is termed as the normal.
• When illustrating a ray diagram, an arrow indicates the direction of the wave's travel.
  • An incident ray is denoted with an arrow pointing towards the boundary.
  • A refracted ray is denoted with an arrow pointing away from the boundary.
• The angles of incidence and refraction are typically denoted as 'i' and 'r' respectively.

A ray diagram for light refracting at a boundary, showing the normal, angle of incidence and angle of refraction

Refraction of Light

• Refraction is the phenomenon that occurs when light transitions from one transparent medium to another.
• As light crosses the boundary between these two media, its rays change direction.
• This change in direction is measured in relation to an imaginary line known as the normal, which is perpendicular to the boundary's surface.
  • When light moves from a less dense medium to a more dense one (e.g., from air to glass), it bends towards the normal.
  • Conversely, when light moves from a more dense medium to a less dense one (e.g., from glass to air), it bends away from the normal.
  • If light travels along the normal (perpendicular to the surface), it does not bend at all.

How to construct a ray diagram showing the refraction of light as it passes through a rectangular block

• When light passes into a denser substance, the rays slow down and bend towards the normal.
• The properties that change during refraction are speed and wavelength; the frequency of waves remains constant. Different frequencies correspond to different colors of light - red has a low frequency, while blue has a high frequency. Despite this, when light refracts, its color remains the same, indicating that the frequency does not change.
• Different frequencies of light account for the various colors we perceive. For instance, red light has a lower frequency compared to blue light.
• During refraction, the color of light does not change. An example of this phenomenon is observing a pencil in a glass of water where the light appears to bend but retains its color, affirming that frequency remains consistent.

Investigating Refraction

Aim of the Experiment

• To explore how light behaves when passing through different shapes like rectangular blocks, semi-circular blocks, and triangular prisms

Variables

•  Independent variable: The shape of the block
•  Dependent variable: The direction of refraction
•  Control variables:
  • Width of the light beam
  • Frequency/wavelength of the light

Equipment List

Resolution of Measuring Equipment

• Protractor: Accuracy of 1 degree
• Ruler: Accuracy of 1 mm

Method

• Place the glass block on a sheet of paper, and carefully draw around the rectangular perspex block using a pencil.
• Switch on the ray box and direct a beam of light at the side face of the block.
• 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.
• 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.
• Draw a dashed line normal (at right angles) to the outline of the block where the points are.
• Remove the block and join the points marked with three straight lines.
• Replace the block within its outline and repeat the above process for a ray striking the block at a different angle.
• 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.

• The diagrams for various shapes will display multiple light ray paths for different angles of incidences (i) where light strikes the blocks. This visualization aids in illustrating how the angle of refraction (r) alters with the angle of incidence.
• To enhance clarity, these paths should be distinctly labeled with (1), (2), (3), or A, B, C for better understanding.
  • Ensure clear labeling of these paths with (1), (2), (3), or A, B, C for improved comprehension.
• Angles i and r are consistently measured from the normal, emphasizing the scientific perspective.
• When light rays penetrate the perspex block, they refract towards the central line.

Illustrative Examples:

• A When a light ray encounters the perspex block, it bends towards the central line due to refraction.
• B The angles of incidence (i) and refraction (r) are pivotal in understanding how light behaves within the perspex block.
• C Clear labeling of the light ray paths aids in tracking the changes in angle as light moves through the block.

Important Notes:

• normal Angles i and r are referenced from the normal, a crucial aspect in studying refraction phenomena.
• towards Light rays, upon entering the perspex block, deflect towards the central line, showcasing the principles of refraction.
  i > r
  This inequality signifies that the angle of incidence is greater than the angle of refraction.

Safety Considerations

• The ray box light could cause burns if touched. 
• In case of burns, run burns under cold running water for at least five minutes. 
• It is also important to avoid looking directly into the light, as this may damage the eyes. 
• During the experiment, stand behind the ray box and keep all liquids away from the electrical equipment and paper.