Playbook
Wave Optics
117 q - 2.96/paper - 19% HARD, spread evenly over Young's double slit, single slit and interference intensity. The one cornerstone that does not cherry-pick.
- Questions in the bank
- 117
- q/paper (2024–25 papers)
- 2.96
- Tagged HARD
- 19%
- Subtopics
- 5
Strand: Cornerstone
When you’ll see it
Two slits and a fringe pattern, a single slit and its central maximum, two sources of stated intensities, or light through a polariser.
How this chapter is tested
117 q at 2.96 a paper, 19% HARD — and the HARD is spread evenly. Young's Double Slit (39 q), Single Slit Diffraction and Resolving Power (34 q) and Interference Intensity (29 q) are each 21% HARD. This is the one cornerstone that does not cherry-pick: own all three pages, or plan to lose three marks a paper.
Young's double slit is fringe width β = λD/d and what moves it: β ∝ λ and D, ∝ 1/d, and immersing the apparatus in a liquid divides it by μ. A thin film over one slit shifts the whole pattern by (μ − 1)tD/d without changing β.
Intensity questions use I = I₁ + I₂ + 2√(I₁I₂) cos φ. With two equal sources the maximum is 4I and the minimum 0; with unequal ones, the ratio I_max/I_min = ((√I₁ + √I₂)/(√I₁ − √I₂))². Polarisation (10 q, 10% HARD) is the quick page: Malus I = I₀ cos²θ and Brewster tan θ = μ.
The sub-skills
The distinct skills inside the chapter, in the order to learn them.
Fringe width and its levers
β = λD/d. In a liquid, β becomes β/μ. A film shifts the pattern by (μ − 1)tD/d.
Interference intensity
I = I₁ + I₂ + 2√(I₁I₂) cos φ. Amplitudes add, intensities do not; take square roots first.
Single-slit diffraction
Minima at a sin θ = nλ; the central maximum is twice as wide as the others, 2λD/a.
Polarisation
Malus: I = I₀ cos²θ; unpolarised light through one polariser loses half. Brewster: tan θ_B = μ.
Traps to expect
Distractor shapes this chapter reuses. The Traps page covers the patterns that cut across chapters.
Adding intensities instead of amplitudes
Two sources of I and 4I give a maximum of 9I, not 5I: (√I + √4I)² = 9I.
Mixing up the double-slit and single-slit conditions
In Young's experiment d sin θ = nλ gives a BRIGHT fringe; in a single slit a sin θ = nλ gives a DARK one. Same equation, opposite meaning.
Learn it before you drill it
This chapter has full teaching notes — foundations, worked examples, self-checks and a per-subtopic mastery checkpoint. Read the notes once, then drill subtopic by subtopic below.
Wave Optics notesDrill every wave optics question
117 questions from the bank, scoped to 5 bundled subtopics.
Drill one subtopic at a time
The 5 subtopics this playbook covers, in catalog order.
Related playbooks
Often paired with this one — the technique, the trap or the taxonomy overlaps. Drill these next.