Consider the case of light being diffracted through two slits, where the wavelength is much larger than the physical scale (in this case, the distance d between the two slits). We determine that the offset in phase between two wavefronts emanating at an angle from the two slits is = d sin .

[NMSU, N. Vogt]

We calculate the combined flux.

which can be expressed as

The intensity of the light is then found, using the small angle approximation and the half angle identities for , to be

If we place a screen a distance f away from the slits (the screen lies perpendicular to the vector labeled r above, where r = f), then we will observe the following pattern. The peaks will appear a distance of = f / d along the screen.

[NMSU, N. Vogt]