Light in Physics O Level and IP Secondary 3 Weight Assessments (WAs)
Download printable cheat-sheet (CC-BY 4.0)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.Consult Us Now
