The purpose of this section is to describe how diffracting plane wave source works and how it differs from Bloch/periodic plane wave source. Well know double slit experiment is used to demonstrate this.

## Discussion and simulation setup

The following analytical formula can be used to calculate the spacing of the interference maxima on the projection plane:

$$ s=\frac{z \lambda}{d} $$

where:

z is the distance of the projection plane from the slits

d is the distance between the slits

lambda is wavelength

Simple calculation shows that the distance between the maxima at 633nm should be approximately 3.05um.

Tip: Reducing the simulation size To minimize the simulation time, it is generally recommended that you do not include large regions of empty space in a simulation. It would be possible to obtain these same results with a much smaller simulation region, by taking advantage of the far field projection functions. This example uses a large simulation region to keep the analysis as simple as possible, even though it is less computationally efficient. |

## Results

The simulation results shown on the figures below demonstrate the diffracting nature of the sources and their constructive and destructive interference. Moreover, the distance between the maxima is ~3.04um, which is well aligned with the analytical result above.