2. The slit

Here You will see an animation of a plane wave breaking through a slit. 

At the borders of the slit a disturbance will happen, leading into the propagation of ringwaves. When the slitwidth is little compared to the wavelength, the two ringwaves will be in the same phase and thus interfering into one ringwave.

You can vary the physical parameters Yourself. One important parameter is the fraction between the wavelength and the slitwidth k = l / d.

Follow the propagation from start by 
 clicking here.