Light in Physics O Level and Integrated Programme (IP) Secondary 3 Weight Assessments (WAs)

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11 Jul 2025, 00:00 Z

TL;DR
"Light" contributes ≈ 15-20 % 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 \(d_o = d_i\)	state, draw, use
Refraction	Snell's Law \(n_1 \sin\theta_1 = n_2 \sin\theta_2\)	calculate, explain
Critical Angle & TIR	\(\sin c = \dfrac{n_2}{n_1}, \space n_1 > n_2\)	determine, justify
Lenses	Thin-lens eqn \(\dfrac1f = \dfrac1u + \dfrac1v\); magnification \(m = \dfrac vu\)	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.

\[ n = \frac{\sin i}{\sin r} \]

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 \(\pu{30 ^\circ}\). Find \(r\): \(\pu{\sin r = \sin30^\circ/1.5 = 0.333 \Rightarrow r = 19.5^\circ}\).

3.2 - Critical Angle & Total Internal Reflection

Condition: ray travels from denser to less dense medium and \(\theta_i < \theta_r\).

Equation: \(\sin \theta_r = \dfrac{n_i}{n_r} \sin \theta_i\). Use three-step check in exams:

Media order: \(\text{glass} \rightarrow \text{air}\)
Calculate \(\theta_r\)
Compare \(\theta_i\) vs \(\theta_r\)

Total Internal Reflection occurs when \(\theta_r = \pu{90 ^\circ}\)

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 \(\rightarrow\) straight line.
Parallel \(\rightarrow\) focus.
Through focus \(\rightarrow\) 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

\[ m = \frac{h_i}{h_o} = \frac{v}{u} \]

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: \(v = \dfrac{c}{n}\).	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 \(\rightarrow\) sharp vary.
"TIR can occur from air to glass."	Needs dense \rightarrow rare.	Apply sin-critical test every time.
"Magnification uses object/image distance only."	Height ratio equals distance ratio; watch sign.	Pair ray diagram with table of \(u, v, h, m\).
"Normal line always drawn at surface."	Must be at point of incidence.	Pencil-dot rule: draw dot then normal.

5 - WA & Revision Blueprint

Window	Action for Students
WA 1 (T1 Wk 6-8)	Practise 3 x 6-mark long questions under 9 min each (1 mark : 1.5 min rule; see Weighted Assessments vs Mid-Year Examination).
Mar break	Run 30-min mixed-topic MCQ sprint; log careless optics errors.
WA 2 (T2)	Sit one full 6091 Paper 2 Section B (optics heavy) - check % finished.
June hols	DIY mock MYE: past ACS(I) or RI Sec 3 paper; aim ≥ 65 %.
EOY promo	Two-week alternating drill: Day 1 theory recall; Day 2 timed paper; Day 3 rest.

6 - Link-Outs to Level-Up Fast

Forces & vectors pre-req - Forces, Dynamics & Free-Body Diagrams
Measurement & spreadsheet labs - Measurement and Uncertainty for IP Physics
Time-management hacks - Developing IP-Level Problem-Solving Habits

7 - Further Reading & Practice

One-Sentence Takeaway

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.

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