I think this is more of a Physics question but certainly has implications in Biology. The phenomenons taking place are diffraction and interference. Of course, couple of things have to first be fulfilled for diffraction to take place the size for the slit relative to the wavelength being one of them etc(if you want a more detailed answer just DM me and I will be happy to explain further). Then relative to the angle formed between the two slits relative to the point on the screen a maxima(“light band”)or a minima(“dark band”) is formed(I am talking about intensity) due to interference of the waves from the two slits. If you have further questions feel free to ask.