SEC G3 Combined Science Physics component K326/K327
CP11: Light
Use ray diagrams and the required reflection, refraction, refractive-index, and converging-lens ideas.
Reviewed for the 2027 cohort on 19 July 2026. The official syllabus remains authoritative for assessable wording and paper details.
Core notes
Light-ray diagrams work when every angle is measured from the normal and each change in direction is tied to reflection, refraction, or a lens.
Reflection
Draw a normal perpendicular to the surface at the point of incidence. The incident and reflected angles are both measured from that normal.
The angle of incidence equals the angle of reflection. Use equal measured angles to construct or check a reflected ray.
Refraction and refractive index
At a boundary, light can change speed and direction. Measure incidence and refraction from the normal, then use the sine ratio for the stated pair of media.
A medium's refractive index is the speed of light in vacuum divided by its speed in that medium. A larger index corresponds to a lower light speed in the medium.
Converging lenses and images
A thin converging lens bends a parallel beam towards its principal focus. Focal length is the distance from the optical centre to that focus.
Describe an image using real or virtual, upright or inverted, and magnified, diminished, or the same size. Work from the rays or information given instead of memorising a table of object distances.
Assessed representations
Reflection, refraction, and converging-lens construction
Formulae and relationships
| Relationship | Use |
|---|---|
| Apply the law of reflection. | |
| Compare incidence and refraction for the stated pair of media. | |
| Relate refractive index to light speed. |
Worked examples
Example 1: Light enters glass from air with and . Find the sine ratio.
- Measure both angles from the normal.
- Calculate .
- The ratio is about 1.51.
Answer: The sine ratio for this boundary is approximately 1.51.
Chapter checkpoint
Use these three moves to organise the topic before attempting a mixed or practical question.
- Construct reflection and refraction diagrams with normals, arrows, and angles measured from the normal.
- Use the stated sine-ratio or refractive-index relationship only for the named media and direction.
- Draw principal rays to locate converging-lens images and describe their nature, orientation, and size.
Official outcome coverage
K326 CP11: 8 mapped outcomes, references CP11(a), CP11(b), CP11(c), CP11(d), CP11(e), CP11(f), CP11(g), CP11(h). Check the official K326 syllabus.
K327 CP11: 8 mapped outcomes, references CP11(a), CP11(b), CP11(c), CP11(d), CP11(e), CP11(f), CP11(g), CP11(h). Check the official K327 syllabus.
The outcome wording is not reproduced here. The relevant official syllabus remains authoritative for exact assessable scope.
Practical and data connection
Verify reflection with measured angles, trace refraction through a glass block, or determine the position and characteristics of an image formed by a plane mirror or thin converging lens. Draw normals, measure from the normal, repeat rays, and explain line thickness, alignment or sharp-focus uncertainty.
Exam traps and retrieval check
Avoid these traps
- Measuring from the surface instead of the normal.
- Using ordinary angles instead of their sines in the ratio.
- Describing image size but not orientation or real-versus-virtual character.
Check from memory
From what line are optical angles measured?
The normal.
What does a converging lens do to a parallel beam?
It brings the rays towards the principal focus.
Name three image descriptors.
Real or virtual, upright or inverted, and magnified or diminished.
Official Combined Science scope
This shared Combined Physics owner serves both K326 and K327. Reflection, refraction, refractive index, converging lenses, focal length, and image descriptions.