The single-slit minimum condition kept apart from the double-slit maximum, the grating equation and its highest order, Rayleigh's criterion, and Malus's and Brewster's laws with their geometry.
Unit 3 carries 8 of the 50 physics marks; with six chapters sharing them, this one averages a little over one (MEC publishes weights by unit, not by chapter). Three scope lines, three small formula families — a sinθ = nλ, 1.22λ/D, and the pair I = I₀cos²θ with tanθ_p = n. Almost nothing here needs a derivation, and almost everything here needs the right formula picked out of a look-alike.
The MEC scope lines are: Concept and conditions of diffraction; single slit and diffraction grating; Resolving power of optical instruments; Polarisation and Brewster's law. The headings below are those points, in MEC's order.
Three ways it is asked: recall (Brewster's law, what Fraunhofer diffraction needs, what a polaroid does); understanding (why sound turns a corner and light does not, why a telescope is built large, why sound cannot be polarised, why polaroid sunglasses are cut for horizontal glare); application (the width of a central maximum, the highest order a grating can show, the intensity through two or three polaroids, a polarising angle from an index).
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