Superposition, the coherence that makes fringes stay still, the two path-difference conditions, and Young's geometry with every symbol in the fringe width defined.
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). It is the cheapest chapter in the unit to secure, because almost everything asked of it comes out of a single equation with four symbols in it — the fringe width — and out of one list, the conditions for sustained interference.
The MEC scope lines are: Concept, conditions and applications; Young's double-slit experiment. The headings below are those points, in MEC's order.
Three ways it is asked: recall (the two path-difference conditions, what coherence means, what a very thin soap film looks like in reflected light); understanding (why interference does not violate the conservation of energy, why two separate bulbs never give fringes, what happens to the fringes when the whole apparatus is lowered into water); application (a fringe width from λ, D and d, a wavelength from a measured fringe spacing, an intensity ratio from an amplitude ratio, the shift caused by a thin sheet in one path).
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