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The passage around this formula

A single reflective surface will not do it. The mirrors used in EUV systems are not a polished metal surface but a stack of dozens of alternating molybdenum and silicon layers, each only a few atomic layers thick, engineered so that the many weak partial reflections at each interface add together in phase. A close analogue to the classical Bragg condition for layered reflectors, $m\lambda \approx 2d\cos\theta$. describes why the stack works at all: at near-normal incidence, $\theta$ is small and $\cos$$\theta$ is close to one, so the first-order condition reduces to roughly $\lambda$ $\approx$ 2d , meaning a bilayer period d of a little under 7 nanometers should reflect strongly somewhere near 13.5–14 nanometers.…

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