Fresnel Diffraction

Observing a Spherical (Light) Wave from $(x,y,z)$ while at $(x^{\prime },y^{\prime },z^{\prime })$ The field in the observation plane, $E(x^{\prime },y^{\prime })$, a distance $z$ from the aperture plane is a sum of Spherical Waves from every point with the aperature

$$E(x^{\prime },y^{\prime })\propto \int \int \underset{\text{Aperature Fn}}{\underbrace{t(x,y)}}\cdot \underset{\text{Electric Field}}{\underbrace{E(x,y)}}\cdot \underset{\text{Spherical Wave}}{\underbrace{\frac{\exp (ikr)}{r}}}dxdy$$ $$\text{where }r=\sqrt{z^2+(x^{\prime }-x{)}^2+(y^{\prime }-y{)}^2}$$

See Fraunhofer Diffraction, the simplified, approximate form