O-Level and SEC G3 Physics K323
P12: Light
Use ray diagrams, reflection, refraction, total internal reflection, and lens relationships with sign and scale checks.
Reviewed for the 2027 cohort on 19 July 2026. The official syllabus remains authoritative for assessable wording and paper details.
Core notes
Light questions combine precise ray construction with reflection, refraction, total internal reflection and converging-lens image formation. Angles are measured from the normal.
Reflection and refraction
For reflection, angle of incidence equals angle of reflection. Both are measured between the ray and the normal. A plane-mirror image is virtual because rays only appear to originate behind the mirror.
Refraction occurs when light changes speed at a boundary and usually changes direction. The ratio (sin i/sin r) is constant for a fixed pair of media. Light bends towards the normal when it enters a medium in which it travels more slowly.
Critical angle and optical fibres
Total internal reflection occurs only when light travels from a higher refractive-index medium to a lower-index medium and the incidence angle exceeds the critical angle. At the critical angle, the refracted ray travels along the boundary.
Optical fibres guide light by repeated total internal reflection. They support high data capacity, low signal loss and immunity to electromagnetic interference, and can carry light through flexible medical endoscopes.
Converging lenses
A thin converging lens brings rays parallel to the principal axis to the principal focus. Focal length is the distance from optical centre to principal focus.
Use principal rays to locate images: a parallel ray refracts through the far focus, a ray through the optical centre continues undeviated in the thin-lens model, and a ray through the near focus emerges parallel. State whether the image is real or virtual, upright or inverted, and magnified, same size or diminished.
Formulae and relationships
| Relationship | Use |
|---|---|
| Apply the law of reflection. | |
| Find refractive index for light entering from air in the usual school model. | |
| Relate refractive index and speed. | |
| Find critical angle for a medium to air. |
Worked examples
Example 1: Light enters glass from air at and refracts at . Find the refractive index.
- Use .
- Substitute .
- Keep angles measured from the normal.
Answer: (napproximately 1.52).
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 refractive index and critical-angle relationships only for the stated media and boundary.
- Draw principal rays to locate lens images and describe their nature, orientation, and size.
Official outcome coverage
K323 P12: 11 mapped outcomes, references P12(a), P12(b), P12(c), P12(d), P12(e), P12(f), P12(g), P12(h), P12(i), P12(j), P12(k). Check the official K323 syllabus.
The outcome wording is not reproduced here. The relevant official syllabus remains authoritative for exact assessable scope.
Practical and data connection
Trace rays or determine focal length with a sharp image criterion, repeated distances, and a clear statement of measurement uncertainty.
Exam traps and retrieval check
Avoid these traps
- Measuring angles from the surface instead of the normal.
- Claiming total internal reflection for light moving into a higher-index medium.
- Drawing a lens image without tracing rays to their actual intersection or backward extension.
Check from memory
What are the two conditions for total internal reflection?
Travel from higher to lower refractive index and incidence angle greater than the critical angle.
Can a virtual image be formed on a screen?
No, because the rays do not actually converge there.
What does a parallel ray do after a converging lens?
It refracts through the far principal focus.
Pure versus Combined scope
Combined Physics revisits part of this core under CP11 Light, but with reduced outcome scope. Combined students should follow the component checklist rather than assume every K323 outcome is assessable.
Shared explanation source
Eclat has a related explanation in its existing IP library. It can help with the shared concept, but its IP extensions and school-sensitive scope are not automatically part of K323. Open the related IP explanation.