Q: What does Light in Physics O Level and IP Secondary 3 Weight Assessments (WAs) cover? A: Everything parents, IP students and O-Level candidates need to ace Light - reflection, refraction, lenses and optical instruments - under Singapore's 6091.
TL;DR "Light" contributes roughly fifteen to twenty percent of O-Level 6091 Paper 2 marks and pops up early in IP Sec 3 WAs. Master three equations (Snell, thin-lens & magnification), five ray-diagram rules, plus the critical-angle test and you can unlock eight common long-answer templates. Use the WA calendar to pace practice, fix misconceptions (e.g., "light slows then bends towards normal") and stack past-paper drills before CCA season.
1 - Why "Light" Matters in IP & O-Level Physics
Weightage: The SEAB 6091 syllabus bundles reflection, refraction and lenses into Sections 17-19, typically 10-12 marks per paper.
Earliest Sec 3 IP WA topic: Many IP schools start waves - light in Term 1, so gaps snowball into optics and EM.
Transfer value: Lenses feed directly into A-Level H2 diffraction - telescope design.
2 - Syllabus Snapshot
Sub-topic
Key learn-and-apply list
Common WA verbs
Reflection
Law, lateral inversion, mirror equation do=di
state, draw, use
Refraction
Snell's Law n1sinθ1=n2sinθ2
calculate, explain
Critical Angle & TIR
sinc=n1n2,n1>n2
determine, justify
Lenses
Thin-lens eqn f1=u1+v1; magnification m=uv
solve, construct ray diagram
Optical Instruments
Eye, camera, projector basics
compare, analyse
3 - Core Ideas & Ray-Diagram Hacks
3.1 - Snell's Law in One Line
When a wavefront meets a boundary, the ratio of sines equals refractive-index ratio.
n=sinrsini
Tip: Memorise as "SIN over SIN" - sketch normal first, label incident and refracted sides.
Worked WA slice Light enters glass n=1.50 from air at 30∘. Find r: sinr=sin30∘/1.5=0.333⇒r=19.5∘.
3.2 - Critical Angle & Total Internal Reflection
Condition: ray travels from denser to less dense medium and θi<θr.
Equation:sinθr=nrnisinθi. Use three-step check in exams:
Media order:glass→air
Calculateθr
Compareθi vs θr
Total Internal Reflection occurs when θr=90∘
3.3 - Thin-Lens Equation & Sign Discipline
Keep to real-is-positive convention (RI-P):
object distance u, image distance v, focal length f measured from lens centre; real images give positive v.
Ray-diagram rules (converging lens)
Through centre → straight line.
Parallel → focus.
Through focus → parallel.
Plot two rays minimum; intersection gives image. Practise with graph paper to keep scale accurate - examiners award marks for clear construction.
3.4 - Magnification
m=hohi=uv
Negative magnification ⇒ inverted. Combine with lens formula to predict image size before drawing.
4 - Top Five Misconceptions (and 2-min Corrections)
Myth
Reality
Fix
"Light speed is constant across media."
Slows in denser medium: v=nc.
Show water-tank laser demo; time-of-flight video.
"Object distance equals focal length for any sharp image."
Only when image at infinity (projector).
Use lens kit: move screen until blur → sharp vary.
Master Snell's Law, Critical Angle and Lens Equations early.
Pair these concepts with scale-true ray diagrams.
The concept of light shifts from route memorisation to an intuition of what to apply these concepts to different problem sets.
