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The passage around this formula

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. Measurement bears this out closely rather than exactly: one recent study of Mo/Si multilayers built for EUV optics reports bilayer periods of 6.97 to 7.07 nanometers across fifty complete bilayers, with reflectivity near 65 percent at near-normal incidence around 10.7 degrees, interface widths of about 0.3 to 0.4 nanometers combining roughness and interdiffusion, and surface roughness held below 0.2…

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