Symbol d

Its job in the formula

d is a part of this expression. Its role is fixed by the surrounding article and by the operations shown in the formula.

Where the article explains it

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.

Learn the underlying idea

A variable is a named place for a value. Its letter is a local label: x can mean position in one formula and a data point in another.

Open the illustrated variables: a letter stands for a value guide →