The aperture of the slit is divided into

points, each of which can be considered the origin of a wave. The wave vectors are summed up at a point on the screen. At a given point each wave has a different distance to travel, which results in a different phase angle

. Let

be the distance between the slit and the screen,

the distance of the point on the screen relative to the optic axis and

the space between one of the points in the slit to the optic axis. The length of a path is then

. Because

it is approximately

. In the case of Fraunhofer diffraction where the slit width

it can be further approximated to

, which means that

and hence

depend linearly on

. The sum of the wave vectors gives a line of constant curvature, a circle. It is completed for the first time in the first minima. When

becomes greater,

cannot be neglected and the phase angle between two neighboring wave vectors is no longer constant. The result of the vector sum is now the so-called

Cornu^{1} spiral, which is an important tool for investigating Fresnel diffraction.